CA1138044A - Trace vapour detection - Google Patents
Trace vapour detectionInfo
- Publication number
- CA1138044A CA1138044A CA000323634A CA323634A CA1138044A CA 1138044 A CA1138044 A CA 1138044A CA 000323634 A CA000323634 A CA 000323634A CA 323634 A CA323634 A CA 323634A CA 1138044 A CA1138044 A CA 1138044A
- Authority
- CA
- Canada
- Prior art keywords
- region
- ions
- sample
- electric field
- tube
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/62—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating the ionisation of gases, e.g. aerosols; by investigating electric discharges, e.g. emission of cathode
- G01N27/68—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating the ionisation of gases, e.g. aerosols; by investigating electric discharges, e.g. emission of cathode using electric discharge to ionise a gas
Abstract
1 00. 1216C
ABSTRACT:
An apparatus for detecting trace quantities of chemical species in the form of vapour by subjecting ions produced in a sample of the mixes gases under flow to an ion repulsive electric field which is directed in the opposite direction to the flow of the gases, compris-ing means such as a fan for drawing the sample through a hollow body, having an inlet, a first region containing means for ionising at least a proportion of the molecules of the sample by a corona discharge, a second region in which the sample is subjected to an oppositely directed ion repulsive electric field and a third region having an electrode for collecting ions of selected polarity. Ions of a selected polarity whose ionic mobilities exceed a value dependent on the strength of the ion repulsive electric field and the velocity of the gas flow are pre-vented from entering the third region. Means such as a resistor may be provided to limit the current of the corona electric discharge to a value not greater than 100 nanoamps.
ABSTRACT:
An apparatus for detecting trace quantities of chemical species in the form of vapour by subjecting ions produced in a sample of the mixes gases under flow to an ion repulsive electric field which is directed in the opposite direction to the flow of the gases, compris-ing means such as a fan for drawing the sample through a hollow body, having an inlet, a first region containing means for ionising at least a proportion of the molecules of the sample by a corona discharge, a second region in which the sample is subjected to an oppositely directed ion repulsive electric field and a third region having an electrode for collecting ions of selected polarity. Ions of a selected polarity whose ionic mobilities exceed a value dependent on the strength of the ion repulsive electric field and the velocity of the gas flow are pre-vented from entering the third region. Means such as a resistor may be provided to limit the current of the corona electric discharge to a value not greater than 100 nanoamps.
Description
~L3~ L4 TRAC~ VA~OUR DETF.CTIO~J
The present invention relates to an apparatus a~d to a rnethod for d~tectlng trace quantities of a chemical species in the form o ~ vapour .in an Gxygen containlng gaseous mixt~e by subjecting ions produc:ed in a sample of the miY~d yases unde~ fl.ow to ~n ion repulsive electric ~ield, in ~hich the ion repuls1.ve electric fiel~ is di.rected in the opposlte di~ection to the flow of i:he gases.
For ~he purpose of th~ present in~rent~on t3~e expression, "chemical species`' refers to vapou~ rnolecules of s~starlces which are capable of forming relatlvely stable ions in ~he preSenC2 of oxygen when under the influence of an appli,e~l electric. ~ield; said ions of the chemical. species having a mobility which is low cornpared ~ith the ~obility of other ions which ma~ be produced in the oxygen containing gaseous rnixture by the applied electri.c fi.~ld.
Include~ in such specles are certain subst.~uted hydrocarbons which ha~e at least one strongly el~ctro~positive or electro-negati.ve atom or at~l group in their mol~cules and also the hea~i.er halogens such as 9 bromine and i.odine.
App~atus cmbodying th~ inventi.on may he used to determine the preserlce of substances in the fcrm o solids, liquids or rni~tures thereof w~lich substallces emi~ ~he a~fore~aid chelNical species in the o~m o~ a vapour at norrnal ternperatl.~e and pressure~ Such substances .includS~, for exarnple$ explosi1v~e subst~lces for e~ample, dynamite~ toY.ic vapours9 for example~ cextain narcot:ic drugs, and pesticidesO
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~ 3~
l'he apparatus may also be employe~l for detecting l~a}~s into the atmosphere of gases which contain the chemic~l species as, for example, a tracer substance, such &t.S a heavy halogen, from pipelines, chemical plant and the like.
It is known that when a sample of atmospheric gases contal~ing traces of chemic:al species of the afvresaid kincl is ionlsed, eOg.
by an applied electric field in the form of a corona discharge, ions are forn\ed from the molecules of the cher~cal species together with ions of the othe~ con~titutents of the atmosph~re~ In general, ~o the various i.ons present will have differing ionic mobilities.
It is also known to use an ion rep~lls.ive elect.rlc field technique to measu~e the mob.ilities of the various ion species created in a corona diccharge, fo.r example as described in a paper by A. Goldman, R, Haug and R. V. Latham published in Journal of Applied Physics, Vol~ 97 No. 6, J~me 1976~ In this paper there is describ~d a method for recording the .inverse mobilit~ spPctra o~ ion species present i.n a continuous gas ~:low situation. The spectra o~tained are from oxygen and nitrogen and fxom air environments. No further explanation is given in regard to the gas compositions used other 20 than confirmirlg that un.specified .impurities may be contained in the ~as l~ixtures.
One object of the present invent.ion .is to provide an apparatus which can detect the pre~ence of chem.ical species of the aforesai.d kind in an oxygen-cvntaj.ning gaseous mixture by s~jecting ions 25 of the aforesald chemical speci.es, when in a continuous gas flo~
situation, to a ion repulsive electric field~ The apparatus and method of the present invention utilizes the fact that the mobllity of ions fornted from such speci.es ~s relati.vely low compared with the mobil.ity Gf ion.s formed from other con3titue~ts of th~ gaseous n~xtuxeO
Another object of the present invention is t:o provide a method G dete~t.lrtg trace t~uantities of chemlcal spt~c.ies of the aforesaid kind in an oxygen-containint~ gaseous mi~ture by selec~ing ions of one polarity of the ch~mi.cal species so that they can ke carried wi~h the gaseolls mlxture throtlt~h an ion repulsive electr~c field~
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~IL38~
According to the present il1ventlon there is provided an apparatus for detectil1g trace quantitj.es o~ chemical species in the ~rm of a vapour contained in an o~ygen-cQntair1i.ng ycl5eous mixture by subjecting ions produced in ~ sample o the mixed gases under 1O~J
to an ion repulsive electric field, said repulsivé fi~ld being directed in the opposite direction to the flow of the gases, characterized in that there is provlded means for drawing the sample through a hollo-,~ body, said hollow bod~ having ser.iall~ arranged an inlet, a first9 a seconc1 and a third interr,al regi.on and an 10 outlet, the fi~:st region containing means or ionising at least a prOpOrtiOIl of the mol.ecules of the samp].e includirlg ~olecules of the chemical. species and m~ans or selecti~g ions of one polarity for travel with the sample into the sPcond reg.ion, means for promoting the ~1C~J of the sample through the second rec~ien as a jet 15 of substantially uniform ~elocity~ means for prod~lcing .in t1~ second region the oppositely directed, ion repulsive electric ield and rr.eans in the third region for collecting ions of selec~ed polarity, the ar~angement being such that, in operati.on, ion~ of the one selected pol.arity whose ionic mobîli.ties exceed a value ~o dependent on the strength of the ion repulsive electric field and the velocity of the cJaS low can be pxe~ented from ente.~1.ng the third region~
In one a~rangement the apparatus is port~le, light in weicJht and operates ~thout a ~ott~ed ~ascupply. Th2 apparatus can be 25 relatively slmple in construction and economi.cal to manufacture and it is particularly advantageous when the apparatus is appli.ed to detecting trace quantities o~ che~ical specles emitted from explosi~e type substances ln that only a short warrn-up kime is re~uired to detect the c:hemical speciesO
30 '~he hollow body may cornprise first and second tubes of an electrically conducting material joined ln end to end relationship by a ri.ng of insulating rnaterial, said irst region being locat.eci within the first tube and the second and third regions bei.11g located within the second t1.ibe~
The fi.rst tube may be provided a~ the end remote fro~ the second t~lbe with c~l encl wall havincJ an .inl.et aperture for admittin(J the sa~ple to the f~^st region and at the ~d adjacent to the secor.d t~lbe wlth a disc of an electrica~ conducting material~ said disc belng pos.i~ioned transverse to the ~Jas flow and having at least one aperture for admlttlng the sarnple to the second region.
The or each ape~ture of t~ disc at the end ad~acen~
the second t~be i.s shaped so that the sample can pass tl~rough the second region in a stream-lined mannex and wi.th ~ substantially uniforrn ~eloci.ty.
The second and third regions may be partially separated by a further d.isc of an electxically conducting material located at a point remote from the ends of the second tube and having at least one aperkure leadir.cJ from the second reg.ion to the third region for ~dmittiny t.he sc~nple to the thi.rd .re~lon9 .said d:isc being in elect~.ical contact wi.th the ~all of the tub~
The means ~or dra~ g the sample through khe hol:low body may be an electrically dr~en fan whi.ch can ~le posi.tioned s~bstclntially at the outlek o~ the hollo~ body.
me means *or ionising at least a propor~ion o~ the molecules of the sample inclllding molecules of the chemical species r~ay be an electrode locateq in the fi.rst region oY a radio acti.ve source located in the first regionq When a radio actiYe source i.s used there is also provided in the fi~st region means for selecting ions of one polarity rom a bi polar ion popul.ation produced by the radio activlty-The tneans for selecting ions of one polarity or travel with the sample into the se~ond region may be an electric ~ield produced b~f a high direct voltage applied to an el.ectxocle in the first: re~ion~
Where ~he :Lonising means i-5 an electrode ~Jhich in operation produces a corona discha~ge it can be advantageous to limit the discharge cu~rent to ~ot more than lOO nanoamps: .in some cases it has been found adYantageous to l.irni.t the current to not more thc~n 30 n~noarnps~ '~le c~rrent--llm.lting can be effected by inserti.ng a ~esistbr be~.~.een a high voltage sou~ce ard the discharge eJ.eetrode, .,, ", . .;, ~:lL3~30~
s When a r~clio active ioni~lng .ource is used the disc hetween the first and second regions i5 electrically insulated fro~ the wall of the first tubc and is m~lnt~ined ~-t a direct current ~?lectri.c potential rcl~ti.ve to the said ~all so that ions of one polarity 5 are attracted towards -the disc and ions o the opposite polarity are repelled from the disc. The further di~sc can be maintained at an.electrical potential relative to the first disc so as to prod~lce an electric ~iel.d in the second reglon which produces a force on the ions of the one selected polari.ty in a direction 10 opposite to that of the flow of tne CJaS 5c~mpl2.
~ wall of the seco~d ~eglon conveniently is connected to a first pole o a direct current electrical power supply of r~egative or positive potential polarity for neyati.ve or positi.ve charged ions respec:tively and the wall of tl~ first region is connected 15 to a second pole of the electri.cal po~/e.~ supply of an apprcp~.iate3y opposite potential to the first pole. The two ~alls may be Malntained at t.he app.ropriate potential by one electric:al po~er supply situated outside the hollow body c~nd conn~cted thereto by electrical c~bles~
An electrode can be provlded in the third region conYeniently axial with the aperture to collect ions o the one .selected polarity~
The electrode may have an lnsulated lead ~hlch passes thrQugh the ~all of the hollow body and ca~ be electrically connected to a source o~ direct current potential to attract the ions o~ selected polc~ri.ty and to means for measu~ing changes in the electrlcal cur.rentO
mus9 in one e~odiment the electrode is co~lected to an input of a current amplifier~ and an output of the ampliier i5 connected to i.ndicating means so as to measure changes of current ~low from the electrode~
me ampl.ifier can be arranged to have a relati~ely low gain w~en tlle current fed to its lnput is constant and a substantically hiqhe~ ~ain to variation~ in its input current~
Conveniently feedback means may be connected between an inverting lnput of the amplifier ~nd a tapping poi.nt on a potentiomete~r chai.n 3s connected between an output of the amplifier and a common ~ail 9 the .
~13~
potentiometer chain including ~ capacitor connected in the sectior~
~etw~en the tapping point and the comrnon rai.l.
The electrode car, he connected to the i.nput of -the amplifier and the common rail connected to th~ source of cl.c. potentlalO
The apparatus may include means for di,schargincJ the capacitor when the current collected by the electrode is rc~ducedO
This emboLi.iment may al.so include means for cL-~paring the potential on the capacitor wi.th the potential at the o~tput of the amplifier and means for discharging the capacitor when il:s 10 potential exc:eeds that at the output of the ampliier~
This arranqement m~y also include ~eans fo~ prevent.ing the capacitor bei.n~ charged ~y pot;er supply surges which can occur when the appar atus is switched on.
A s~i'cch mealls ~Ih.ich is effective wher t.he apparatus is switc}led 15 on to connect a low i.mpedance path across the capacitor and ~o discos~ect the said path whe3l th~ power supply to the apparatus has reached a steady state may also ~L~ included.
AccoL^dillg to a further aspect of the ir~ventlon there is also provided a rnethod or detfcting trace f~antities of chem-cal spe~ies ~ is~ the orm of a vapour in an oxygen-containing gaseous rRi~ture by subject.ing ions proclucecl in a sample of the mixed gases under flo~"
to an io~ repulsl~e e1ectric field, said repulsive ield being directed in the opposi.te direction to the flow of gases ch~lracterised by the steps of drawing the gaseous mixture through the hollow boly 25 as hsreinbefore defined wherein a proporti.on of t,he r,lolecules of the che~ical sp~cies are~ionised in the first region ~nd ions of one polarity a~e selected for onwc~,d t~avel wi.th the sample of the gaseous mixiu.~e, to pass with the gas flow at a su~stantlally uniform velocity through the second region wherein is esta~lish2cl an electric fi,eld effective to urge ions of the selected polarity in a direetion opposite to that of the gas flow and through to a third ~egio~ to which the ele,ctric field does not extend, and detectillg icns whose ionic rnobil.ities ~re suficiently low so that th~y are carried by the gas ~lo~.~ through the electric field in the second region~ In the method a mix.t;~3.re of ~tmospher;.c gases cont~itling trace quanti.t,.i.es of the ch~nical ~pec.i.es ~nay be usedO
~313~
Compositions which yen~rate a smoke can h~ detected~ for example, by ~u~igati.n~ candl~s or fum.i~at.i.ng powdel-s which e~it a smoke of compounds when burnlngO The compol~nds emi.tted may b* Mixed ~th the products of a co~b~stible mixture e.g. a fuel such as, sugar 5 or t~C~ in a sround forr.l~ together withl substances to maintain cor~bustion, such as, ar~monium nitrate or potassium chlorate, and~or substances to delay combustion9 such as, kaolin9 bentvnite and/or silicic aciclO
Compositions contai.ning trace ci~anti.t:;es of the cher~ical species 10 may he detected *rom sprayed pesticidal preparations. It is kno~m, ln the control of noxious organisms ? in agr.iculture 9 horticulture or forestry thak large areas to be treated can be sprayed ~lith aqueous sol.~ltions or diipersions of pesticides l~e amo~lt of liquid used can vary ~idely and may result in an appli.cation of from 15 ~0 litres to lO00 l.itres per hectare. ~hese solutions may be applied hy spYaylllg frorn cJround machi.nes or fro;n a~rcraft and freq~lently are applied froin Ultra-lc)w-volume concentrates. Such preparations cJenerally ~.real~ up as a ~nist of very srnall drGps on application and the drops may have diarn~ters o only about lO0 ~m 20 whilst still in the vapG~ri.sed fo~n~ Trace quantities of th~
aforesaid chem1.cal speci.es may be readily detected by the meth~l and apparatus a~cording to the present inventionO
, The presen~e o aerosol composltions l~ay a.7.so be dete~mined 25 particularly ~h~ the ae~osol compositions containing the chemical species are prepared iri the conventiona7 manner and contain the active substa~ce 9 with a solvent and a volatile fl~lid as ~he propellant, for example~ chloro~fluoro derivati.~fes of methane or ethare~
Ernbodiments of the .invention and the manne~ in which it is to be perfo~med will n~ be desribed ~ith reference to the accompanying draw.in(7s, in which:-Figure ~ 1.. a sectional drawj.ng of a detection apparat-ls e~odylrlg the invention, E`igure 2 is a circuit diagraln er~odyin~ th~ invention~
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~. _, ,.,.. , . ,, ~ . ... . ..
4~
~i9ure 3 is a ;no.re detai.led schemat:ic dlac;ram o an ampli.fier unit ~o~ use in tl~e apparatus of Flgure 1, and Figure 4 .is a sectional draw.i.ng of a further embodiment of the invention.
Referring first to ~ u~e 19 the det-,ection apparatus cornprises a c~enerally cYllndrical bocly 1 formed by coaYlal tt~bes 2 and 3 of electrically-conduc~ing materi.al such as iron, copper or alumlnium or alloys thereo joined in end-to-end relat.i.onsrlip by a ring 4 - of an insulating m~ter.ial. The tube 2 is pro~ided at its free 10 end with an end~wall 5 of conduct.ive material provi.d~d ~i.th a central ape~^ture 6~ An inlet nozzle or probe 7 coaxial with the apert~re 6 is proY:ided on the outer ~ace of the end wal.l 5~
The noz~,le 7 may be of insulcltin~ materi.al or of a condu~.t.ing mat:er:i.al such as a metal, i.n which case it ls insulatecl frorn the end wall 5 by an insulating spacer ri.ng 8~
At the end of the tube 2 adjacen'.: the tube 3 there is provided a sept~n 9 corl,p~isinc3 a mel:al di.sc ha~:ing an a~ial apert~re lO, said septurn 9 being elec.tr.ically connected to the wall. of the tube 2.
A baffl~ 11 is located at a point intermedi.a-te the ends of the tube 3 ~hi.ch laf1e 11 comprises eiti)er a metall.i,c disc haYing at least one aperture 12 or a sheet o metall.ic gau~,e or a grld like struct~re the circumference of the b~l~fle c~onvenlently b~ing electrically connected to the ~all of the tuhe 3~
A fan 13 driYen by an electric r.~ot.o~ 14 is located with.in the tube 3 adjacent its open end and is effecti~ to draw a sample of the atmosphere in to the ap~a~~atus through the noxzle 7 and thence successively thL^ough a first reglon 15 wlthin the body 1 defi~ed by the end-~all 5 and the sepliurn 9, through a second regi.on 16 de.~ined by th~ septurn ~ d thc bafle 117 thro~lgh a third region 17 defi,ned by the baffle 11 and the fan 13 and to expel the sample through the open end of the tub~ 3.
The aperture lO of the se2tum 9 ~ean be profil~l so that th~
sample passes throu~}l the ~-cgio~ 16 as ~ single smooth ,et of substantiall~ unj.~oA:m Yelocity, as indicated hy the arro~J 18~
Suitc~:le~ cont~u's fo~ the a~arture lO are shG~n in ~rit.ish .5tanda~ds .
~3~0~
Speci~ication No. 1042 at p. 1~1. In an alternative arrangement, the septum 9 has a plurality of apertures each regularly profiled so -that the sample passes through the region 16 as a corresponding plurality of smooth jets of uniform velocity. In order to smooth the flow of the upstream side of the septum 9, a grid 19 having a plurality of parallel passageways is pro-vided in the aperture 6.
A pointed electrode 20 is located in the region 15, being supported by a lead 21 which passes through an insulator 22 mounted in the wall of the tube 2. Outside the tube 2, the lead 21 is connected via a resistor 71 to one side of a high voltage supply unit 23 whose other side is connected to a common earth point. The supply unit 23 produces a sufficiently high voltage V3 for exam-ple, 3000V, for a corona discharge to occur at the elec-trode 20 and to cause ionisation of a proportion of the molecules (including molecules of the chemical species aforesaid) which constitute the sample being drawn through the region 15.
Other suitable high voltages used for producing an ionising corona discharge may be between 2000V and 5000V.
The resistor 71 has a value ~ chosen so that the current ID of the corona discharge is less than 100 nano-amps and preferably is less than 30 nanoamps.
The current ID depends on the output voltage V3 ofthe supply unit 23, the value R of the resistor 71 and also to some extent on the shape of the pointed electrode 20, w~ich affects the striking potential of the discharge. I
This shape may change in use, due for example to erosion of the electrode, with consequent changes in the value of ID.
A further source of changes in ID is the supply unit 23, which may conveniently comprise an EHT generator unit energised from a low voltage source such as a 12V battery, which as will be described more fully hereinaf~er, may also be used to energise other parts of the apparatus. Such EHT
generator units are well-known to those skilled in the art and need not be further described herein. The output volt-age V3 of such a unit may vary from its nominal value ~3~4~
.
as the state of charge of the batte~y varies with use.
For these reasorls, or a particulax nomil-al val~e o~ V3 and a gi~en value of R, the discharge current ID rnay fall within a range o~ v~lu~s.
Typlcal ranges for the value of ID for various values of R
and a norninal value of 3000V for V3 are as ollows:-?
R = O I~ a 2 to 5 microc~nps R - 50 Gigohms I = 2V to 30 nanoamps R - 100 Gigohms I~ = 10 ko li nanoamps Pxeferablys t,herefore~ the value of the resistor 71 is in the range 50 to 10~ Gigohms~
hs the mo].ecules of the said chemical spec:ies are presenl; in the sample only in small concentra'cion it ~.ould be difficult to achieve appreciable le~els of ionisation of sp~cies mole~ules by direct ionisati,on. To do this the extrem .ly powexful discharge n~:eded to ionise thr~ species molecules wo~ld ~lso cause a massl~re i.onisat1.on o~ the a.ir gasesO '~nis is a~roided by allowing the discharge to ionise a proport,ion o~ thr.~ air gases so that a charge exchange react.ion can ~ake plac~ :in which charge exchange reclcticn the charges on ai~ ions are transf,~red to molecules of the said species. Considering negati~e io~s, ionisation of molecules of said species present .in low çoncentration can take place by this means if the molecules o~ said specles ha~e a greater electron affir~ty than an~r of the molecules p~esent ~n relat;.~ely massive nur~ers, .i.eO the air molecules. ~lectrons present in a ~s will rapidly transfer to the molecules with the greater electron af~irlityO
~ loreover, i two chernical species havi.ng d.if~ering electron affin.ities axe present in the sa~ple in trace ~uantities9 each of, for exarnple9 1 in 10 mole per mole5 ion pop~lations of both may be created, provided that both ha~e yreate~ electron affinities . ~ ~han any of t,he molec~les present i~ rnassive nutt~berst because collisi.on~ bet~leen molec.ules of the ~,pecl~s havllly the higher electron affirli.ly w.ith 10~s of the speci.es haYincJ the lo~;er elect.ror , ", ,, .. ,.~ .. , . . ~.. ,, ., .. .. .. .... ,, .. . . .. . ,, , .. . . ~ . . ........... . . . . ; - . -.
~:L38~
affinity (isl ~/hi.ch the ion could be ~obbed Gf an electron) ~ill, due to the low concertration o, both species, by much less frequent than coll.isions bet~en molerules of e.ither speci.es and ai.r ions, .in w}-ich molecules of either species may be ionlsed~
Si~ilar considrerations apply to posi.ti.ve i.on populations~
Because of the tendency of ionic charge to rnig~ate to a molecule .
o c~reater affinity, a sicJn.ificant proport,ion o the molecules of a che~nical spec~es of the type sper.,ifie~l prr.~sr*nt in an air sample dra~n through the region 15 becosnes ion.ised.
Consider for exarnple th~ ca~e o~ a species presen-t in air at a nu~erical conrent.ration (molecule~; per molecule) of 1 i.n 1~ .
'l`he corona d.i.scha~ge from the electrode 20 may typically cr,eate a primary (i.eO air) ion density of 1 ion per l,0 mol~cllles o~ air in the, region lSo hssu~e that one tenth c.) the species sr,olecules present become.s ioni.sed by ch~rge e~change r~actlorlsO ~ne nurr.erical concent~ation of sp~cles ions is thus 1 in 10 snolecules. ~.ne concentratiQIl of the prisnary ions renl~in~ substarltial.ly unchanged ak 1 in 10 mol~cule~s. Hence the species ions cos~prlse 1 in 10 of all ions present~ The detection problem has thus been cha~ged 20 f~om detecting one rnolec~lle in 10 to tllat of de~te_t.i.ng one in 10~ that is an improyem.ent o 10 t~r~es.
Although ~,he primary ~ai.r) iorl population generated by the corona discharge in tlle region 15 ~lI initially be blpolar, the very high negative potential o~ the el.e~ctrode 20 relative to the tube ~, and the resulting i.ntense ~i~ld in its YiCi.nity causes positive ion.s to migrate to ~.he electrocle where khey are nelltrali~ed, Thus s~stanti.-~lly c,nly negative primary ions travel ~ith the air flow totia.rds the aperture 10 o-E the septum 9. Dur:lng the travel of the negatlve pri~a~y ions tr~wards the septurn7 a proportion of the specles molecules pxesent in the air flo~!t~ill become ionised by th~ charge-exch~nge reactlo~s as describecl herelnbe~ore, ~us there will arri.~e at l:he ar,eL^ture 10 a rnixtllre o~ negatjve primar.y (a~r) ions and necJative seconci..~^y .ions created ~rorn the molecules of t~ chemic21 species~
~.. .. .
~.3~
In the absence of an electr:ic field7 thes~ lons would be carried by the jet flo~- 18 acro.ss the region 16 and through the baf*le 11 into the thi.rd re~.ion 17. In th.is region 17 there is provided an elec:trode 2~ ~hi.ch may compri.se a ~ire~ a grid or a plate and ~Ihich is connected by a lead 25 ~1hich passes through a~ insulator 26 in the wall oF the tl~be 3 to an input term.inal 27 of a curJ^ent ampl.ifier unit 28. 'I~he ampli.f.ier unit 2~ rnay have a common t~.nminal connected to a point at a relatiYeAly low positive potential ~1.th respeet to tlle ~ube 3, or exc~nple 5 voltsS as .indi.cated schematically 0 in Figure 1 b~ the battery 29, so th~t the input tcrn~nal 27 2nd with it the el~ctrode 2~, is also held positive with respæct to the tube 3h The electrc~3e 2~ is thus effective to attract neyati~e i.ons enteriny the region l~/o Such ions collected by the electrode 24 will produce a ch~nge in the input c-~rellt to the amplifi~r w~it 2~J D
The ~esulting change i.n the c,utput current of the ~pli.fier. Ullit rnay be obse~vecl by ~e~ns o a ~eter 30 and/or may be utlllsed to tri.gger an alarm circuit 31 e-ff ectlve to produce a signal, such as an aud.ible or a visi.hle signall A so~ce of potenti.al, typicdl~.y~ 3~0 ~olts, indicated schematically by a ba~te~y 32 i5 connected between the tube 2 and the tu~ 3 so tha1; 'ch~ tube 31 znd ~ith it the septu~ 9 is, positive wi.th respect to the tu~-~ 2 and in p~rticular to the b2ffle llo An electric field is therefore pxoduced in the regi.on 16~ Althouyh there are ,'.ringing effects neclr the wall of the body 1~ the f:i.eld in the vic.inl~ of ~he ~et 18 is substantially parallel to the a~i.s .~f the jet ar~ is effective to urge negative ions in a direction opposite to tllat of the jet ~low~
~ e strength of khe ~lecti:ic field in the Yicin.ity o th~
jet 18 is cle~ter~nined by the separa~ion be~ JF.en the septum 9 and th~
3~ baffle 11 and the potential ~ of the source 3,7~ This m~,y be for e~ample, 300 volts per cr~l. I`or a given field strength ions ~hose ionic IDobilit.:l.es have a certain cri.ti.cal vallle ~rill be ~riven by the fie~ld at ex~ctly the veloci~ of the ~et flol~, b~t in the opposite directionO A sultable jet flow velocity is between ~ to 400 cm per seccn~ Such ions ~ill re~na.in stat.iona~ so ar c~s trotion par~:L~ to the jet axi.s is conce~r,ed. '~'2ir XClndOrl trans~ersF.
. .
~333~
motion may move them laterally into a ~ec~iol~ o~ slowe~ gas f1o~
and the fielcl in this r~ ion Inay then drive t.hem back to the septum 9. Cons~quently~ higher mob.ility ions are substanti.ally p,revented by tile field from reaching the baffle 11 and enteriny th~ region 17.
Ions ~hich have mobilities }-i.gher than the critical value are pl^evented from enterin~ t}-e re~ion 17, since they are driven back by the fi.elc'i ill the region 16 at a velocity greater than that o the jet flow.
Ions with mobilit:i.es lo~e~ than the critical value are given a velocity by the ield in the region 16 lower than the veloc.i.ty o the jet flo~0 Altllough some such ions may be carried by ~andom transverse rnotions into regions ~/here the velocity of the gas flow is less than the velocity imparted to t:hern by the fi~ld, in general those i.ons ~hose mo~,~ilities are lower than the critical value ~Jill be carried by the jet flow across the region 16, albeit at a veloclty less t.han that of the jet 10w9 an-l w.111. enter the region 17 and be collected by the elect~o~e 24 It will be seen that for a given fielt~ strength and a giver geometry in the xegion 16, ions ~ith mob.i.l.ltles equal to o.
~ exceeding a critical value have no possibllit;y of reaching the electrod~ 2~ For .ions of mobilities 1ower than t'ne cri.tical value there is a i.nite po.ssibility which i.ncreases as tlle mob.ility of the i.on and hence the ti~ne ta~en for i.t to transverse the region 16, decreases.
In use the value of the potential V of the source 32 is chosen to produce a field strength such as to exclude nearly all the p~imary ~air) ions from the re~ion 17 but to allow heavler, less rnobile ions~ such as those p~oduced from chemical species of the type specified to enter the ~egi~ and reach the collector electrode 24. Such ions n~anifest themselves as arl i.nc~ease in the output current of the ampli~ier 2~ which may be o~ser~ed on the meter ~0 ~nd which may be employed to triggex the alarm circuit 31~
A sa.mple of air d~a~ through th* appara~us the meter 30 35 pro~luces a su~st Lntially constarLt lV~! cur.rent (backgrourLd cuL-rent~
1~
which is incli~ated by meter 30. .Tf deslred the rneter may be offset eith~r mechan.ically or electr::ically to give a zero readin~ un(~e, thes~ con~litior~s~
When a sample of a.ir inclu~ing trase quantities of chernical spec.i.es of the type specifie(l is dra~ through the apparatus, the meter reading ~il]. increase hy an amount: depcnden~ on the concentl.ation of the species.
me ~larrll circui.t 31 rnay he arranged to produce the alarrn signal when the output current of the amplifier uni.t 28 exceeds ~0 a thresilold level which is greater than the back~ound current level~
The apparatus 50 far described with r~ference to Fl.gure 1 is adapted to detect the pres~nce of chen~ical. species whi.ch will forrd n~gati.ve ions by charge exchange reactions~ ~ny positive . ions which may be ~ormed in the region 15 are attrac~ed to the discharge electrode 20 and do not proceed into the regjon 16.
~o adapt th~ ayp.lrat-ls to detect species ~hich wi.ll for~ positive ions it is neces~3L^y merely to reverse the pol~ri.ties of t.he high voltage supply un;.t 23 and of the voltacJe sources 29 and 32.
The arnpli~ier unit 28 .Is preferably of a type having a h.igh 20 dcgree of d~co stability ~lo~ drift) and a rel.atively high gain1 so that small changes in its lnput c~rent such as may be caused by the. presellce of ~e.ry low concentrations of chelnical spe-,i.es of the type specifi.ed may produce significant changes in t~e a~,pli..fier output cur~ent which are not rnasked by variatlons of the background current caused by ~rit in the amplifier unit~
F~gure 2 ;ll.ustxates a basic arrangement and Figure 3 is a more detailed scl-ematic diagxam of a current ampli.ier or use as the amplifier uni.t 28. Referring ~irst to Figu~e 2~ the ~exmi.zlal 27 i s con~lected to an .invert.ing input 33 of an operational am~lifier 30 341 ~Jhose non~irL~Jert.inc~ input 35 is connec~ed to a co~on rail 36O
Resistors 37 and 38 and a capacitox 39 are connected in series between an output terminal 40 of the Gperati.onal ~.,plifier 34 and the commo~ xail 36, A feedback resistox 41 is connected bet~/een the Jllnction of the re.sistors 37 and 38 and thc lnvexting lnpu~ 33.
35 T~pically a Burr P,rclwn ampli~ier l~pe 35i~7 AM may be employed as . . .
~38~
the amplifier 34. 1~ re~;stors 37, 3~ ~md 41 may have .respecti~e value.s of 33M ohm, 330K. ohm and 500M~ ohm and the capaclt.or 39 n~.ay have a value of 47 ~F.
Since at zero frequency the capaci.tor 39 .is ef~3ct.ivel.y ~n open clrcuit, it will be seen t~t the arrangement has a d~c~ gain of unity, and consequently has nigh d.c~ stabi.lity (lo~l drift~.
At frequencies for ~thich the imped~nce of the capacitor 39 is small colnpared ~/ith that of the resistor 3~t the gain t~.l be dete~.ined b~f the ratio of the resistors 37 and 38~ For the values ,0 ~i~en by ~lay Or e~3mple here.inbefore it will be seen that the galn of the arrangement will be 100 for requenc~es dotrn to a sma].1 fraction o~ 1 HæO
Certain difficulties arise if the arrangement of ~igurf~ 2 i.s employed in the amplifie~- unit 28~ If an atmospheric sample contca~nir.g 5 chemical species of the type specified i5 dra-~ through the apparatus, the resulti.ng neg~ti-~e ions reaching the electrod~ 2a till cau~e a nega~i~e going current pulse to be applied to the input terminal 27.
At the leadir.~ edge of t~ pulse, the output termi.nal 40 ttill go positi~e9 charging the capacitor 3~ At the erld of the pulse9 20 the output terminal 40 tlill not immediate'y return to its ~est ~al~e (bac~ground cu~rerlt) but t~ill be held positi~e by the cha39e on the capacitor 39. Consequently in thi.s embod.iment the apparatus can be unable to respotld to a second sanlp~e until such time as the charge on the capaci.tor 39 prcduced by a irst s~nple has leaked ~
U5ing the componer.t yalues cited, and neglecting any inte~nal leakaye in the capac.itor 39, i.t will be seen that the apparatus may req~lire se~eral hours to recove~
A sirnilar ef~ect may occ~r when the apparatus is first switc1le~
on~ when th~3 in.itial surye o current rnay leave the capacitor 39 with a positive charge, and it will be necessary to wait until this c.h~rye has leaked away before the apparatus becomes operati.onal~ ~e arrancJeme.nt of Figur~ , pro~ides a means of overcor~inq bo~h these defects~
~Irring to F:iqure 3, in ~;Jhich lntegers already descr.ibed a.re .3~3a4~
accor~ed the sc~me reference num2rals ar; :in E`i.guL-e ? 9 it w.ill h~
seen that the ol)erati.onal ~Inpli.fier 3~, the resls'Lors 37, 3~3 an~ 41 ~nd t~? caE~dci.t.or 3'J are co~ectAd as descri~ed with referen~e to Figure 2~ The common rail 36 is conr~ect~d to ~he negative bu~ o~ a d.c. supply via a zener rlic~e ~3~ e positi.Ye s~pply tel-minal of the ~mpl.ifier 34 is connected to the posi.ti~e bus 44 of the supply via a s~;itch 45~ The co~non rail. 369 and with i.t the input 33 oF the amp3.ifi~r 34 is therefore he].d positive 1:o the negatit~e bus ~2 by the~ ~ener Y~ltage Vl of the diocle 43 (tvpically 5V)~ The bus 42 is comlected to ground and to the tube 3 (E`i~lr~ l). Thus the electrode 24 is helcl at a positive potential Vl with respect to the tube 1 and the zener diode 43 comprises the voltaye source 23 o~ Figuxe 14 A fl~ther amplifier ~6 has its inve~ti.n~J input 47 connected to the output terminal 40 of the c-~pli.~iex 34~ A non~in~ertiny input 48 of the arnplifi.er 46 i5 connect.ed via a. r~sistor 49 to the j~Cti.011 50 of the resistor 38 and the c.:dp~citor 39.
A diode 51 is connected het~een the outp~t. terr~linal 52 of the amplifier 46 and .its non~invertiny input 4~, the arran~ement beiny such that ~.~hen the input 47 is posit:ive rel~tive to the illpUt 48 the diode 51 i~s non~conducting. When the input 4~ is positive relative to the input 47, i.~e~ the capacitor 39 is charyed positi~e rclatiYe to the output 40 o~ the c~nplifier 34 9 the cliode 51 is conductiYe and the capaci.tor ~9 iS ~lscharged via the resistor 49, which typi.cally has a value of: 33K~ oh~ Although the capac.itor 39 b~comes charged when a n~ative~go.ing current pulse is applied to the input terminal 27~ it now rapldly becomes dischc~ged aft~r the end of the pulse~ so that the apparatus can respond to a ~urther s~nple~
~o prevent the capacito.r 3~ h~comi.ng cha~ged when the switch 45 is first cl.osed, a p n~p transistcr: 53 is p~ovided, wlth its collector connected t.o the junction SO. T}le base of the transi.stor 53 is connectc-d via a resistor 5~ t:o the junction 55 of a resistor ~6 and a capaci-tor 57 whicl-~ are conrleted in series between the po~;:itlve bus 44 and the negci~:i.ve. bu.~ ~ 2 0 ~ res.is~.or ~3~
58 is connect~l bet~een the posi.tive bus ~4 and the emitter of the transistor 53. A diode 5~ S its an~YIe conne~cte~d to che emitter of the transistor 53 and it:s cathode to t}~e common rall 36.
~.~en the switch ~5 is closed, thee junction 55 and with i.t the base of the transistor 53 will init.i.ally be at the potential o~ the negative bus 42~ e ~ransistor 53 ~lill there~ore co~ciuct and will clamp the ~nct.ion 50 at the potential o the con~!non xail 36.
me potenti.al of the junction 55 will ri.se ~s the capacitor 57 charges v.la the resi.s-tor 56, eventu~ cut-ting ofr the transistor - 1,0 53 and .so xem~i.n~j the clarnp from the junction 5C.'2 Th~ Yalues of the resistor 56 and the CapacitQr 57 are chosen so tllat the trans~stsr 53 remà:~ns conduc~tive at least until the ampll*ier 34 has reache.d its steady~state conditi.on follo~ing the c].osure Oc the sw.itch 45, thus preventirlg the accumulation of any cha~ge on the~
c~apacitor 39 due 'o switching~on urges~
To facilitate se~ting up o* the ste~d~~st~te condi.tio~l of the~
amp~.ifier unit (hacl~ground currer~t) a po~enti.ornete~ 6S~ may be provided ~ One end of the~ potentiometer m~y be connected to the pos.itive bus ~ia the resistor 5~ and the otller end to the common 20 rail 36 via a resls~or 61, theS res.isto~s 58 ancl 61 serving to restri.ct the ~o1tac3.r-.~ e~Y.cut~slon t~Jhich rn-~f he oht.a.ired by adjust!n~nt.
of the position of the slider 62. 'l~e sli.der is connected via a resistor 63, typically of 33 M.o}~n to the junc-ti.on o~ the feeclback resistor 41 ~nd the rt-~sistors 37 and 33. By adjustmen~ of t~.e potentiometex 60 Ole backc~round current level may be set to a de~i~ed valueO
~ ne neqatlve ~2) and ~e positi~!e (fi4) buses ma}~ be connected to coxLespondil)g tenninals o~ a battery 70 o fOL eY~mple 12V
output~ m e battery 70 ~ay also be employed to enercJ.i.s2, by the on-off switch ~5, the motor 14 ar.d ~he voltage sources 23 and 32 tF.igure 1). The voltatJe source 23~ which provides the hic~
voltage neces.sary to produce the corGna discharge i.n the region 15, may conveniently comprise an ~,HI' generator circuit such as is hell kno.~l to those skilled in the art, ant~ need not be described 35 furtheL hex^e1xls Siml:Lcirly t:he~ volt2~cJe source 32 may- comprise an ~3L3~
~.~
E!ll` generator or d~c. to ~Cr eonverter of known type.
InvesticJations haYe sl^lown that ~h~ .influence o~ e~ternal air curr~nts i5 cJt.-eatly retiuce(i if the sample is eYLhau.stecl to atmosphere i.n a re~on adjclcent t-o, md subject to substanti.ally the same exter~l condltions as, the lnlet nozzle and to this encl the body 1 is surroundecl by an outer casiny 64~ ~le casing 64 has an end wall 65 spacecl fro;n the open outlet end o~ the tube 3 so as to provide a passac;eway ~or the flo~J of air e~austed by the -fan 13 into an amlular space 66 between the body 1 and the outer casi.ng 64 and thence ~o atmosphere via an annular aperture 67 surro1md:i.ncj the inlet ~nd o the body 19 ThC~ 10w path of the exhaust a.ir i.s i~icated by the arrows 63, ~9.
Convenient:Ly9 the battery 70, the voltac~e sources 23 e~nd 32, the arnplifier unit 28 and the alarm un:i.t 31 may be located wlthin the outer casincJ 6~ to produce a po.rtable~ ha~ld~held apparatu.s.
In an alternative arrangeln~nt, pri.nary ionisation may be caused by a sou~ce of ionising radiation (22' Yic~ure 4~ e~g~ a ~--parl:icle emltt.er such as ~mericium 241, or B~-particle emitters s~lch as Nlckel G3 or Tritium, located i~ a cont~ er 20' (Figure 4) with:in 2U the re~ion 15. Sine the prl~ary ion population so produced would be bi-polar, it is also necessary to protide in the reglon 15 an elcct~ic fleld effecti~e to select ions of one polarity for onward travel with the ai.r 10w and to prevent .ions o the opposit.e polarity rom approach.ing the septu~ 9. One method o~ achievi.ng this is to i.nsulate J~he sept~ 3 ~ uxe ~) by a ring 41 of insul.ating materi.al from the tube 2 and to apply a suitable potential tn rebetween~
The ~eptum 9 (Figure 1) is also insulated f.rom tube 2 when the pri.mary ionisation is caused by a so~ce o ioni.si.ng radi.ation located i.~ a container (nGt shown) in the region 15 The electrocle 20p lead 21, insulator 22 and supply unit 23 ~ay then be omitted~
The contai.ner 20' containing a sou~ce of ionisi.ng rad.iation m~y be in~erted into the re~ion 15 through the ape~ture in the wal.l 2 ~Fiyure 1.) indicated for t:he .insulator 22~ A s~,~ ~ arrange~c-:nt can be used in the app~ratu~.represented by Ficlure 4 and in this case the aperture (not sh~l:rl) wi.~l pass thro~lg7l ~oth w~11 2 a~d wall 6~o ..... .. .
~3~
'rlliS ar~ally~em~llt miti~ates problems of p.re~enting le~ka~e of atomic radiati.on f rom the app,~ratusO
~ pp.li.cations o~ the present lnventlon include~ antJ.~terroList measures such ~AS the screening for explo.siv~ substances of travellers and luggage at airway terminals and similar locations, ancl searching for explosive substances gener-ally. ~pparatus e~mbodying the invention m.cly also be empl.oyed as a detecto.r in yas chromatography.
A furthe~ application is ill the tracin~ ~f lealcs in equ-~pr;,ent such as pip~wor~ pressure vessels1 chemical plant and the like, wherein a tr~cex: sl~bstanc~ such AS a heavy halogen is intr~luced .~.nto the equipr,knt under test and the appara'cus ls rno~ed over the e.~terior of the ec~ Tnent to de-tect any escape of the trAcer substanceO
In a t.~pic.al e~:p2riment with an en~odim2nt o-f the appaLatUS
as described with re~rence to F~gure 1, the cliameter of the jet or.ifice 10 was approximately 6 mm and the rate of ~as 10w through the apparatus was approxima~ely 10 litres~nlin~te. The potential V3 o the corona di.schar~e electrode 20 was 3~Y, th~ p~tenti~-nl Vl of the collector electrode 2~ was SV and the strength o the repulsive electrlc field in the rec;i.on ].6 was 300VJcm. 'rhe resistor 71 was shorted out~ so that ID was in the range 2~5 micro-c~mps o ~ ' , Whel~ nor~al atmospheric air was clra~n through the apparatus the currer~t collected by the electrode ~ SiOe~ the back~round current) was 1 picoamp.
25. A sample o a nlixture o nitroglycerine vapour in air, havincJ a known concentration of 1 part by welght of ni~roglycerinevapour in 1~ parts by weight of the mixture was then drat~n through the apparatus9 and t.he current of the colJ.ect:or electrode 2~
then became S pico ~ps, the rise of ~ picoarrlps bei.ng caused by t}~e presence of the nitrocJlycerine ~apGllr i.n the sample~
me experiment was then rep~ated Wit}l the resisto~ 71 having a ~alue of Gi.go~ms in circuit so th~ the discharc3e current did no~ exceed 3C~ nanoanlps.
~ le back.cJround cur~erlt was then 0~3 picoamp, and the current when the nltrog~ ri.ne sam?le was drawn thrGugh the apparatus was loO picc)amp~
The reduc-t,iorl in background current i5 due to the lesser noise produced by -the limited current corona dis-charge.
In a further series of experiments, sampl~s of various obtruding substances and also samples of various exp],osives were presented, firs-tly to an apparatus in which the resis-tor 71 was shorted out and secondly to an apparatus in which the resistor was in circuit and had a value of 50 Gigohms. All other parameters of the apparatus were main-tained constant throughout the experiments. The resultsobtained are shown in the following table, in which read-ings of the meter 30 are listed against the substances.
The meter had an arbitrary linear scale from 0 to 100 (= full scale deflection).
Substance R = 0 R = 50 Gigohms Solvents PC~ O O
Methanol 0 0 Ethanol 25 8 Xylene 90 Nitrobenzene 25 0 White Spirit 75 0 _olishes Mansion* 35 0 Cherry Blossom Shoe* 75 0 Kiwi Shoe* 75 0 Brasso* 90 Turtle Wax Car Polish* 90 0 Treble Wax Car Polish* 50 0 Duraglit* 99 Toiletries Avon Blue Blazer After-Shave* 15 2 Nail Varnish 75 0 Tabac Eau de Cologne* 10 0 Chanel No. 5 Eau de Cologne* 35 0 Just Musk Perfume* 0 * trade mark products Substance R=0 R=50 Gigohms . .
Toiletries Indian Summer Perfume* 0 4 Azuree Perfume* 35 0 5Miss Balmain Perfume* 25 0 Scandale Spray* 5 0 Old Spice After-Shave* 5 2 Brylcreem* 50 3 Nivea Cream* 10 0 10Old Spice Talc for Men* 25 0 Brut After-Shave* 50 0 Sunsilk Conditioner* 35 2 Rears Hand Cream* 25 0 Ultra-Brite Toothpaste* 90 3 15Disinfectants Savlon* 70 0 Pine*
Miscellaneous ._ Petrol 15 0 20Tin of paint (Polyurethane~ 90 16 Horseradish Sauce 15 0 Ground Almonds 5 0 Curry Powder 0 0 Chanel No. 5 on Skin Wet* 25 0 " " " " " " after 5 mins 50 2 " " " " " " " 15 mins 35 5 ll ll ll ll ll ll " 25 mins 50 4 Explosives Needle from Geligni-te flask 75 99 Double-Based Powder 90 80 Plastic Igniter Cord (Slow) 25 0 Siesmex H.E.* 15 0 Polar Ajax* 75 85 Minespex* 50 36 *trade mark products ~31~V~
Substal~ce R = OR = 50 Gigohms Wincoal (A(* 50 90 Water Gel O O
. _ . .
Temperature: 25C
All samples offered as opened con-tainers (except where stated).
Applications of the present invention include, the screening for explosive substances of travellers and lug-gage at airway terminals and similar locations, and search-ing for explosive substances generally. Apparatus embody-ing the invention may also be employed as a detector in gas chromatography. A further application is in the tracing of leaks in equipment such as pipework, pressure vessels, chemical plant and the like, wherein a tracer substance such as a heavy halogen is introduced into the equipment under test and the apparatus is moved over the extexior of the equipment to detect any escape of the substance.
* trade mark products
The present invention relates to an apparatus a~d to a rnethod for d~tectlng trace quantities of a chemical species in the form o ~ vapour .in an Gxygen containlng gaseous mixt~e by subjecting ions produc:ed in a sample of the miY~d yases unde~ fl.ow to ~n ion repulsive electric ~ield, in ~hich the ion repuls1.ve electric fiel~ is di.rected in the opposlte di~ection to the flow of i:he gases.
For ~he purpose of th~ present in~rent~on t3~e expression, "chemical species`' refers to vapou~ rnolecules of s~starlces which are capable of forming relatlvely stable ions in ~he preSenC2 of oxygen when under the influence of an appli,e~l electric. ~ield; said ions of the chemical. species having a mobility which is low cornpared ~ith the ~obility of other ions which ma~ be produced in the oxygen containing gaseous rnixture by the applied electri.c fi.~ld.
Include~ in such specles are certain subst.~uted hydrocarbons which ha~e at least one strongly el~ctro~positive or electro-negati.ve atom or at~l group in their mol~cules and also the hea~i.er halogens such as 9 bromine and i.odine.
App~atus cmbodying th~ inventi.on may he used to determine the preserlce of substances in the fcrm o solids, liquids or rni~tures thereof w~lich substallces emi~ ~he a~fore~aid chelNical species in the o~m o~ a vapour at norrnal ternperatl.~e and pressure~ Such substances .includS~, for exarnple$ explosi1v~e subst~lces for e~ample, dynamite~ toY.ic vapours9 for example~ cextain narcot:ic drugs, and pesticidesO
, .
~ 3~
l'he apparatus may also be employe~l for detecting l~a}~s into the atmosphere of gases which contain the chemic~l species as, for example, a tracer substance, such &t.S a heavy halogen, from pipelines, chemical plant and the like.
It is known that when a sample of atmospheric gases contal~ing traces of chemic:al species of the afvresaid kincl is ionlsed, eOg.
by an applied electric field in the form of a corona discharge, ions are forn\ed from the molecules of the cher~cal species together with ions of the othe~ con~titutents of the atmosph~re~ In general, ~o the various i.ons present will have differing ionic mobilities.
It is also known to use an ion rep~lls.ive elect.rlc field technique to measu~e the mob.ilities of the various ion species created in a corona diccharge, fo.r example as described in a paper by A. Goldman, R, Haug and R. V. Latham published in Journal of Applied Physics, Vol~ 97 No. 6, J~me 1976~ In this paper there is describ~d a method for recording the .inverse mobilit~ spPctra o~ ion species present i.n a continuous gas ~:low situation. The spectra o~tained are from oxygen and nitrogen and fxom air environments. No further explanation is given in regard to the gas compositions used other 20 than confirmirlg that un.specified .impurities may be contained in the ~as l~ixtures.
One object of the present invent.ion .is to provide an apparatus which can detect the pre~ence of chem.ical species of the aforesai.d kind in an oxygen-cvntaj.ning gaseous mixture by s~jecting ions 25 of the aforesald chemical speci.es, when in a continuous gas flo~
situation, to a ion repulsive electric field~ The apparatus and method of the present invention utilizes the fact that the mobllity of ions fornted from such speci.es ~s relati.vely low compared with the mobil.ity Gf ion.s formed from other con3titue~ts of th~ gaseous n~xtuxeO
Another object of the present invention is t:o provide a method G dete~t.lrtg trace t~uantities of chemlcal spt~c.ies of the aforesaid kind in an oxygen-containint~ gaseous mi~ture by selec~ing ions of one polarity of the ch~mi.cal species so that they can ke carried wi~h the gaseolls mlxture throtlt~h an ion repulsive electr~c field~
. .
~IL38~
According to the present il1ventlon there is provided an apparatus for detectil1g trace quantitj.es o~ chemical species in the ~rm of a vapour contained in an o~ygen-cQntair1i.ng ycl5eous mixture by subjecting ions produced in ~ sample o the mixed gases under 1O~J
to an ion repulsive electric field, said repulsivé fi~ld being directed in the opposite direction to the flow of the gases, characterized in that there is provlded means for drawing the sample through a hollo-,~ body, said hollow bod~ having ser.iall~ arranged an inlet, a first9 a seconc1 and a third interr,al regi.on and an 10 outlet, the fi~:st region containing means or ionising at least a prOpOrtiOIl of the mol.ecules of the samp].e includirlg ~olecules of the chemical. species and m~ans or selecti~g ions of one polarity for travel with the sample into the sPcond reg.ion, means for promoting the ~1C~J of the sample through the second rec~ien as a jet 15 of substantially uniform ~elocity~ means for prod~lcing .in t1~ second region the oppositely directed, ion repulsive electric ield and rr.eans in the third region for collecting ions of selec~ed polarity, the ar~angement being such that, in operati.on, ion~ of the one selected pol.arity whose ionic mobîli.ties exceed a value ~o dependent on the strength of the ion repulsive electric field and the velocity of the cJaS low can be pxe~ented from ente.~1.ng the third region~
In one a~rangement the apparatus is port~le, light in weicJht and operates ~thout a ~ott~ed ~ascupply. Th2 apparatus can be 25 relatively slmple in construction and economi.cal to manufacture and it is particularly advantageous when the apparatus is appli.ed to detecting trace quantities o~ che~ical specles emitted from explosi~e type substances ln that only a short warrn-up kime is re~uired to detect the c:hemical speciesO
30 '~he hollow body may cornprise first and second tubes of an electrically conducting material joined ln end to end relationship by a ri.ng of insulating rnaterial, said irst region being locat.eci within the first tube and the second and third regions bei.11g located within the second t1.ibe~
The fi.rst tube may be provided a~ the end remote fro~ the second t~lbe with c~l encl wall havincJ an .inl.et aperture for admittin(J the sa~ple to the f~^st region and at the ~d adjacent to the secor.d t~lbe wlth a disc of an electrica~ conducting material~ said disc belng pos.i~ioned transverse to the ~Jas flow and having at least one aperture for admlttlng the sarnple to the second region.
The or each ape~ture of t~ disc at the end ad~acen~
the second t~be i.s shaped so that the sample can pass tl~rough the second region in a stream-lined mannex and wi.th ~ substantially uniforrn ~eloci.ty.
The second and third regions may be partially separated by a further d.isc of an electxically conducting material located at a point remote from the ends of the second tube and having at least one aperkure leadir.cJ from the second reg.ion to the third region for ~dmittiny t.he sc~nple to the thi.rd .re~lon9 .said d:isc being in elect~.ical contact wi.th the ~all of the tub~
The means ~or dra~ g the sample through khe hol:low body may be an electrically dr~en fan whi.ch can ~le posi.tioned s~bstclntially at the outlek o~ the hollo~ body.
me means *or ionising at least a propor~ion o~ the molecules of the sample inclllding molecules of the chemical species r~ay be an electrode locateq in the fi.rst region oY a radio acti.ve source located in the first regionq When a radio actiYe source i.s used there is also provided in the fi~st region means for selecting ions of one polarity rom a bi polar ion popul.ation produced by the radio activlty-The tneans for selecting ions of one polarity or travel with the sample into the se~ond region may be an electric ~ield produced b~f a high direct voltage applied to an el.ectxocle in the first: re~ion~
Where ~he :Lonising means i-5 an electrode ~Jhich in operation produces a corona discha~ge it can be advantageous to limit the discharge cu~rent to ~ot more than lOO nanoamps: .in some cases it has been found adYantageous to l.irni.t the current to not more thc~n 30 n~noarnps~ '~le c~rrent--llm.lting can be effected by inserti.ng a ~esistbr be~.~.een a high voltage sou~ce ard the discharge eJ.eetrode, .,, ", . .;, ~:lL3~30~
s When a r~clio active ioni~lng .ource is used the disc hetween the first and second regions i5 electrically insulated fro~ the wall of the first tubc and is m~lnt~ined ~-t a direct current ~?lectri.c potential rcl~ti.ve to the said ~all so that ions of one polarity 5 are attracted towards -the disc and ions o the opposite polarity are repelled from the disc. The further di~sc can be maintained at an.electrical potential relative to the first disc so as to prod~lce an electric ~iel.d in the second reglon which produces a force on the ions of the one selected polari.ty in a direction 10 opposite to that of the flow of tne CJaS 5c~mpl2.
~ wall of the seco~d ~eglon conveniently is connected to a first pole o a direct current electrical power supply of r~egative or positive potential polarity for neyati.ve or positi.ve charged ions respec:tively and the wall of tl~ first region is connected 15 to a second pole of the electri.cal po~/e.~ supply of an apprcp~.iate3y opposite potential to the first pole. The two ~alls may be Malntained at t.he app.ropriate potential by one electric:al po~er supply situated outside the hollow body c~nd conn~cted thereto by electrical c~bles~
An electrode can be provlded in the third region conYeniently axial with the aperture to collect ions o the one .selected polarity~
The electrode may have an lnsulated lead ~hlch passes thrQugh the ~all of the hollow body and ca~ be electrically connected to a source o~ direct current potential to attract the ions o~ selected polc~ri.ty and to means for measu~ing changes in the electrlcal cur.rentO
mus9 in one e~odiment the electrode is co~lected to an input of a current amplifier~ and an output of the ampliier i5 connected to i.ndicating means so as to measure changes of current ~low from the electrode~
me ampl.ifier can be arranged to have a relati~ely low gain w~en tlle current fed to its lnput is constant and a substantically hiqhe~ ~ain to variation~ in its input current~
Conveniently feedback means may be connected between an inverting lnput of the amplifier ~nd a tapping poi.nt on a potentiomete~r chai.n 3s connected between an output of the amplifier and a common ~ail 9 the .
~13~
potentiometer chain including ~ capacitor connected in the sectior~
~etw~en the tapping point and the comrnon rai.l.
The electrode car, he connected to the i.nput of -the amplifier and the common rail connected to th~ source of cl.c. potentlalO
The apparatus may include means for di,schargincJ the capacitor when the current collected by the electrode is rc~ducedO
This emboLi.iment may al.so include means for cL-~paring the potential on the capacitor wi.th the potential at the o~tput of the amplifier and means for discharging the capacitor when il:s 10 potential exc:eeds that at the output of the ampliier~
This arranqement m~y also include ~eans fo~ prevent.ing the capacitor bei.n~ charged ~y pot;er supply surges which can occur when the appar atus is switched on.
A s~i'cch mealls ~Ih.ich is effective wher t.he apparatus is switc}led 15 on to connect a low i.mpedance path across the capacitor and ~o discos~ect the said path whe3l th~ power supply to the apparatus has reached a steady state may also ~L~ included.
AccoL^dillg to a further aspect of the ir~ventlon there is also provided a rnethod or detfcting trace f~antities of chem-cal spe~ies ~ is~ the orm of a vapour in an oxygen-containing gaseous rRi~ture by subject.ing ions proclucecl in a sample of the mixed gases under flo~"
to an io~ repulsl~e e1ectric field, said repulsive ield being directed in the opposi.te direction to the flow of gases ch~lracterised by the steps of drawing the gaseous mixture through the hollow boly 25 as hsreinbefore defined wherein a proporti.on of t,he r,lolecules of the che~ical sp~cies are~ionised in the first region ~nd ions of one polarity a~e selected for onwc~,d t~avel wi.th the sample of the gaseous mixiu.~e, to pass with the gas flow at a su~stantlally uniform velocity through the second region wherein is esta~lish2cl an electric fi,eld effective to urge ions of the selected polarity in a direetion opposite to that of the gas flow and through to a third ~egio~ to which the ele,ctric field does not extend, and detectillg icns whose ionic rnobil.ities ~re suficiently low so that th~y are carried by the gas ~lo~.~ through the electric field in the second region~ In the method a mix.t;~3.re of ~tmospher;.c gases cont~itling trace quanti.t,.i.es of the ch~nical ~pec.i.es ~nay be usedO
~313~
Compositions which yen~rate a smoke can h~ detected~ for example, by ~u~igati.n~ candl~s or fum.i~at.i.ng powdel-s which e~it a smoke of compounds when burnlngO The compol~nds emi.tted may b* Mixed ~th the products of a co~b~stible mixture e.g. a fuel such as, sugar 5 or t~C~ in a sround forr.l~ together withl substances to maintain cor~bustion, such as, ar~monium nitrate or potassium chlorate, and~or substances to delay combustion9 such as, kaolin9 bentvnite and/or silicic aciclO
Compositions contai.ning trace ci~anti.t:;es of the cher~ical species 10 may he detected *rom sprayed pesticidal preparations. It is kno~m, ln the control of noxious organisms ? in agr.iculture 9 horticulture or forestry thak large areas to be treated can be sprayed ~lith aqueous sol.~ltions or diipersions of pesticides l~e amo~lt of liquid used can vary ~idely and may result in an appli.cation of from 15 ~0 litres to lO00 l.itres per hectare. ~hese solutions may be applied hy spYaylllg frorn cJround machi.nes or fro;n a~rcraft and freq~lently are applied froin Ultra-lc)w-volume concentrates. Such preparations cJenerally ~.real~ up as a ~nist of very srnall drGps on application and the drops may have diarn~ters o only about lO0 ~m 20 whilst still in the vapG~ri.sed fo~n~ Trace quantities of th~
aforesaid chem1.cal speci.es may be readily detected by the meth~l and apparatus a~cording to the present inventionO
, The presen~e o aerosol composltions l~ay a.7.so be dete~mined 25 particularly ~h~ the ae~osol compositions containing the chemical species are prepared iri the conventiona7 manner and contain the active substa~ce 9 with a solvent and a volatile fl~lid as ~he propellant, for example~ chloro~fluoro derivati.~fes of methane or ethare~
Ernbodiments of the .invention and the manne~ in which it is to be perfo~med will n~ be desribed ~ith reference to the accompanying draw.in(7s, in which:-Figure ~ 1.. a sectional drawj.ng of a detection apparat-ls e~odylrlg the invention, E`igure 2 is a circuit diagraln er~odyin~ th~ invention~
.. .. .
~. _, ,.,.. , . ,, ~ . ... . ..
4~
~i9ure 3 is a ;no.re detai.led schemat:ic dlac;ram o an ampli.fier unit ~o~ use in tl~e apparatus of Flgure 1, and Figure 4 .is a sectional draw.i.ng of a further embodiment of the invention.
Referring first to ~ u~e 19 the det-,ection apparatus cornprises a c~enerally cYllndrical bocly 1 formed by coaYlal tt~bes 2 and 3 of electrically-conduc~ing materi.al such as iron, copper or alumlnium or alloys thereo joined in end-to-end relat.i.onsrlip by a ring 4 - of an insulating m~ter.ial. The tube 2 is pro~ided at its free 10 end with an end~wall 5 of conduct.ive material provi.d~d ~i.th a central ape~^ture 6~ An inlet nozzle or probe 7 coaxial with the apert~re 6 is proY:ided on the outer ~ace of the end wal.l 5~
The noz~,le 7 may be of insulcltin~ materi.al or of a condu~.t.ing mat:er:i.al such as a metal, i.n which case it ls insulatecl frorn the end wall 5 by an insulating spacer ri.ng 8~
At the end of the tube 2 adjacen'.: the tube 3 there is provided a sept~n 9 corl,p~isinc3 a mel:al di.sc ha~:ing an a~ial apert~re lO, said septurn 9 being elec.tr.ically connected to the wall. of the tube 2.
A baffl~ 11 is located at a point intermedi.a-te the ends of the tube 3 ~hi.ch laf1e 11 comprises eiti)er a metall.i,c disc haYing at least one aperture 12 or a sheet o metall.ic gau~,e or a grld like struct~re the circumference of the b~l~fle c~onvenlently b~ing electrically connected to the ~all of the tuhe 3~
A fan 13 driYen by an electric r.~ot.o~ 14 is located with.in the tube 3 adjacent its open end and is effecti~ to draw a sample of the atmosphere in to the ap~a~~atus through the noxzle 7 and thence successively thL^ough a first reglon 15 wlthin the body 1 defi~ed by the end-~all 5 and the sepliurn 9, through a second regi.on 16 de.~ined by th~ septurn ~ d thc bafle 117 thro~lgh a third region 17 defi,ned by the baffle 11 and the fan 13 and to expel the sample through the open end of the tub~ 3.
The aperture lO of the se2tum 9 ~ean be profil~l so that th~
sample passes throu~}l the ~-cgio~ 16 as ~ single smooth ,et of substantiall~ unj.~oA:m Yelocity, as indicated hy the arro~J 18~
Suitc~:le~ cont~u's fo~ the a~arture lO are shG~n in ~rit.ish .5tanda~ds .
~3~0~
Speci~ication No. 1042 at p. 1~1. In an alternative arrangement, the septum 9 has a plurality of apertures each regularly profiled so -that the sample passes through the region 16 as a corresponding plurality of smooth jets of uniform velocity. In order to smooth the flow of the upstream side of the septum 9, a grid 19 having a plurality of parallel passageways is pro-vided in the aperture 6.
A pointed electrode 20 is located in the region 15, being supported by a lead 21 which passes through an insulator 22 mounted in the wall of the tube 2. Outside the tube 2, the lead 21 is connected via a resistor 71 to one side of a high voltage supply unit 23 whose other side is connected to a common earth point. The supply unit 23 produces a sufficiently high voltage V3 for exam-ple, 3000V, for a corona discharge to occur at the elec-trode 20 and to cause ionisation of a proportion of the molecules (including molecules of the chemical species aforesaid) which constitute the sample being drawn through the region 15.
Other suitable high voltages used for producing an ionising corona discharge may be between 2000V and 5000V.
The resistor 71 has a value ~ chosen so that the current ID of the corona discharge is less than 100 nano-amps and preferably is less than 30 nanoamps.
The current ID depends on the output voltage V3 ofthe supply unit 23, the value R of the resistor 71 and also to some extent on the shape of the pointed electrode 20, w~ich affects the striking potential of the discharge. I
This shape may change in use, due for example to erosion of the electrode, with consequent changes in the value of ID.
A further source of changes in ID is the supply unit 23, which may conveniently comprise an EHT generator unit energised from a low voltage source such as a 12V battery, which as will be described more fully hereinaf~er, may also be used to energise other parts of the apparatus. Such EHT
generator units are well-known to those skilled in the art and need not be further described herein. The output volt-age V3 of such a unit may vary from its nominal value ~3~4~
.
as the state of charge of the batte~y varies with use.
For these reasorls, or a particulax nomil-al val~e o~ V3 and a gi~en value of R, the discharge current ID rnay fall within a range o~ v~lu~s.
Typlcal ranges for the value of ID for various values of R
and a norninal value of 3000V for V3 are as ollows:-?
R = O I~ a 2 to 5 microc~nps R - 50 Gigohms I = 2V to 30 nanoamps R - 100 Gigohms I~ = 10 ko li nanoamps Pxeferablys t,herefore~ the value of the resistor 71 is in the range 50 to 10~ Gigohms~
hs the mo].ecules of the said chemical spec:ies are presenl; in the sample only in small concentra'cion it ~.ould be difficult to achieve appreciable le~els of ionisation of sp~cies mole~ules by direct ionisati,on. To do this the extrem .ly powexful discharge n~:eded to ionise thr~ species molecules wo~ld ~lso cause a massl~re i.onisat1.on o~ the a.ir gasesO '~nis is a~roided by allowing the discharge to ionise a proport,ion o~ thr.~ air gases so that a charge exchange react.ion can ~ake plac~ :in which charge exchange reclcticn the charges on ai~ ions are transf,~red to molecules of the said species. Considering negati~e io~s, ionisation of molecules of said species present .in low çoncentration can take place by this means if the molecules o~ said specles ha~e a greater electron affir~ty than an~r of the molecules p~esent ~n relat;.~ely massive nur~ers, .i.eO the air molecules. ~lectrons present in a ~s will rapidly transfer to the molecules with the greater electron af~irlityO
~ loreover, i two chernical species havi.ng d.if~ering electron affin.ities axe present in the sa~ple in trace ~uantities9 each of, for exarnple9 1 in 10 mole per mole5 ion pop~lations of both may be created, provided that both ha~e yreate~ electron affinities . ~ ~han any of t,he molec~les present i~ rnassive nutt~berst because collisi.on~ bet~leen molec.ules of the ~,pecl~s havllly the higher electron affirli.ly w.ith 10~s of the speci.es haYincJ the lo~;er elect.ror , ", ,, .. ,.~ .. , . . ~.. ,, ., .. .. .. .... ,, .. . . .. . ,, , .. . . ~ . . ........... . . . . ; - . -.
~:L38~
affinity (isl ~/hi.ch the ion could be ~obbed Gf an electron) ~ill, due to the low concertration o, both species, by much less frequent than coll.isions bet~en molerules of e.ither speci.es and ai.r ions, .in w}-ich molecules of either species may be ionlsed~
Si~ilar considrerations apply to posi.ti.ve i.on populations~
Because of the tendency of ionic charge to rnig~ate to a molecule .
o c~reater affinity, a sicJn.ificant proport,ion o the molecules of a che~nical spec~es of the type sper.,ifie~l prr.~sr*nt in an air sample dra~n through the region 15 becosnes ion.ised.
Consider for exarnple th~ ca~e o~ a species presen-t in air at a nu~erical conrent.ration (molecule~; per molecule) of 1 i.n 1~ .
'l`he corona d.i.scha~ge from the electrode 20 may typically cr,eate a primary (i.eO air) ion density of 1 ion per l,0 mol~cllles o~ air in the, region lSo hssu~e that one tenth c.) the species sr,olecules present become.s ioni.sed by ch~rge e~change r~actlorlsO ~ne nurr.erical concent~ation of sp~cles ions is thus 1 in 10 snolecules. ~.ne concentratiQIl of the prisnary ions renl~in~ substarltial.ly unchanged ak 1 in 10 mol~cule~s. Hence the species ions cos~prlse 1 in 10 of all ions present~ The detection problem has thus been cha~ged 20 f~om detecting one rnolec~lle in 10 to tllat of de~te_t.i.ng one in 10~ that is an improyem.ent o 10 t~r~es.
Although ~,he primary ~ai.r) iorl population generated by the corona discharge in tlle region 15 ~lI initially be blpolar, the very high negative potential o~ the el.e~ctrode 20 relative to the tube ~, and the resulting i.ntense ~i~ld in its YiCi.nity causes positive ion.s to migrate to ~.he electrocle where khey are nelltrali~ed, Thus s~stanti.-~lly c,nly negative primary ions travel ~ith the air flow totia.rds the aperture 10 o-E the septum 9. Dur:lng the travel of the negatlve pri~a~y ions tr~wards the septurn7 a proportion of the specles molecules pxesent in the air flo~!t~ill become ionised by th~ charge-exch~nge reactlo~s as describecl herelnbe~ore, ~us there will arri.~e at l:he ar,eL^ture 10 a rnixtllre o~ negatjve primar.y (a~r) ions and necJative seconci..~^y .ions created ~rorn the molecules of t~ chemic21 species~
~.. .. .
~.3~
In the absence of an electr:ic field7 thes~ lons would be carried by the jet flo~- 18 acro.ss the region 16 and through the baf*le 11 into the thi.rd re~.ion 17. In th.is region 17 there is provided an elec:trode 2~ ~hi.ch may compri.se a ~ire~ a grid or a plate and ~Ihich is connected by a lead 25 ~1hich passes through a~ insulator 26 in the wall oF the tl~be 3 to an input term.inal 27 of a curJ^ent ampl.ifier unit 28. 'I~he ampli.f.ier unit 2~ rnay have a common t~.nminal connected to a point at a relatiYeAly low positive potential ~1.th respeet to tlle ~ube 3, or exc~nple 5 voltsS as .indi.cated schematically 0 in Figure 1 b~ the battery 29, so th~t the input tcrn~nal 27 2nd with it the el~ctrode 2~, is also held positive with respæct to the tube 3h The electrc~3e 2~ is thus effective to attract neyati~e i.ons enteriny the region l~/o Such ions collected by the electrode 24 will produce a ch~nge in the input c-~rellt to the amplifi~r w~it 2~J D
The ~esulting change i.n the c,utput current of the ~pli.fier. Ullit rnay be obse~vecl by ~e~ns o a ~eter 30 and/or may be utlllsed to tri.gger an alarm circuit 31 e-ff ectlve to produce a signal, such as an aud.ible or a visi.hle signall A so~ce of potenti.al, typicdl~.y~ 3~0 ~olts, indicated schematically by a ba~te~y 32 i5 connected between the tube 2 and the tu~ 3 so tha1; 'ch~ tube 31 znd ~ith it the septu~ 9 is, positive wi.th respect to the tu~-~ 2 and in p~rticular to the b2ffle llo An electric field is therefore pxoduced in the regi.on 16~ Althouyh there are ,'.ringing effects neclr the wall of the body 1~ the f:i.eld in the vic.inl~ of ~he ~et 18 is substantially parallel to the a~i.s .~f the jet ar~ is effective to urge negative ions in a direction opposite to tllat of the jet ~low~
~ e strength of khe ~lecti:ic field in the Yicin.ity o th~
jet 18 is cle~ter~nined by the separa~ion be~ JF.en the septum 9 and th~
3~ baffle 11 and the potential ~ of the source 3,7~ This m~,y be for e~ample, 300 volts per cr~l. I`or a given field strength ions ~hose ionic IDobilit.:l.es have a certain cri.ti.cal vallle ~rill be ~riven by the fie~ld at ex~ctly the veloci~ of the ~et flol~, b~t in the opposite directionO A sultable jet flow velocity is between ~ to 400 cm per seccn~ Such ions ~ill re~na.in stat.iona~ so ar c~s trotion par~:L~ to the jet axi.s is conce~r,ed. '~'2ir XClndOrl trans~ersF.
. .
~333~
motion may move them laterally into a ~ec~iol~ o~ slowe~ gas f1o~
and the fielcl in this r~ ion Inay then drive t.hem back to the septum 9. Cons~quently~ higher mob.ility ions are substanti.ally p,revented by tile field from reaching the baffle 11 and enteriny th~ region 17.
Ions ~hich have mobilities }-i.gher than the critical value are pl^evented from enterin~ t}-e re~ion 17, since they are driven back by the fi.elc'i ill the region 16 at a velocity greater than that o the jet flow.
Ions with mobilit:i.es lo~e~ than the critical value are given a velocity by the ield in the region 16 lower than the veloc.i.ty o the jet flo~0 Altllough some such ions may be carried by ~andom transverse rnotions into regions ~/here the velocity of the gas flow is less than the velocity imparted to t:hern by the fi~ld, in general those i.ons ~hose mo~,~ilities are lower than the critical value ~Jill be carried by the jet flow across the region 16, albeit at a veloclty less t.han that of the jet 10w9 an-l w.111. enter the region 17 and be collected by the elect~o~e 24 It will be seen that for a given fielt~ strength and a giver geometry in the xegion 16, ions ~ith mob.i.l.ltles equal to o.
~ exceeding a critical value have no possibllit;y of reaching the electrod~ 2~ For .ions of mobilities 1ower than t'ne cri.tical value there is a i.nite po.ssibility which i.ncreases as tlle mob.ility of the i.on and hence the ti~ne ta~en for i.t to transverse the region 16, decreases.
In use the value of the potential V of the source 32 is chosen to produce a field strength such as to exclude nearly all the p~imary ~air) ions from the re~ion 17 but to allow heavler, less rnobile ions~ such as those p~oduced from chemical species of the type specified to enter the ~egi~ and reach the collector electrode 24. Such ions n~anifest themselves as arl i.nc~ease in the output current of the ampli~ier 2~ which may be o~ser~ed on the meter ~0 ~nd which may be employed to triggex the alarm circuit 31~
A sa.mple of air d~a~ through th* appara~us the meter 30 35 pro~luces a su~st Lntially constarLt lV~! cur.rent (backgrourLd cuL-rent~
1~
which is incli~ated by meter 30. .Tf deslred the rneter may be offset eith~r mechan.ically or electr::ically to give a zero readin~ un(~e, thes~ con~litior~s~
When a sample of a.ir inclu~ing trase quantities of chernical spec.i.es of the type specifie(l is dra~ through the apparatus, the meter reading ~il]. increase hy an amount: depcnden~ on the concentl.ation of the species.
me ~larrll circui.t 31 rnay he arranged to produce the alarrn signal when the output current of the amplifier uni.t 28 exceeds ~0 a thresilold level which is greater than the back~ound current level~
The apparatus 50 far described with r~ference to Fl.gure 1 is adapted to detect the pres~nce of chen~ical. species whi.ch will forrd n~gati.ve ions by charge exchange reactions~ ~ny positive . ions which may be ~ormed in the region 15 are attrac~ed to the discharge electrode 20 and do not proceed into the regjon 16.
~o adapt th~ ayp.lrat-ls to detect species ~hich wi.ll for~ positive ions it is neces~3L^y merely to reverse the pol~ri.ties of t.he high voltage supply un;.t 23 and of the voltacJe sources 29 and 32.
The arnpli~ier unit 28 .Is preferably of a type having a h.igh 20 dcgree of d~co stability ~lo~ drift) and a rel.atively high gain1 so that small changes in its lnput c~rent such as may be caused by the. presellce of ~e.ry low concentrations of chelnical spe-,i.es of the type specifi.ed may produce significant changes in t~e a~,pli..fier output cur~ent which are not rnasked by variatlons of the background current caused by ~rit in the amplifier unit~
F~gure 2 ;ll.ustxates a basic arrangement and Figure 3 is a more detailed scl-ematic diagxam of a current ampli.ier or use as the amplifier uni.t 28. Referring ~irst to Figu~e 2~ the ~exmi.zlal 27 i s con~lected to an .invert.ing input 33 of an operational am~lifier 30 341 ~Jhose non~irL~Jert.inc~ input 35 is connec~ed to a co~on rail 36O
Resistors 37 and 38 and a capacitox 39 are connected in series between an output terminal 40 of the Gperati.onal ~.,plifier 34 and the commo~ xail 36, A feedback resistox 41 is connected bet~/een the Jllnction of the re.sistors 37 and 38 and thc lnvexting lnpu~ 33.
35 T~pically a Burr P,rclwn ampli~ier l~pe 35i~7 AM may be employed as . . .
~38~
the amplifier 34. 1~ re~;stors 37, 3~ ~md 41 may have .respecti~e value.s of 33M ohm, 330K. ohm and 500M~ ohm and the capaclt.or 39 n~.ay have a value of 47 ~F.
Since at zero frequency the capaci.tor 39 .is ef~3ct.ivel.y ~n open clrcuit, it will be seen t~t the arrangement has a d~c~ gain of unity, and consequently has nigh d.c~ stabi.lity (lo~l drift~.
At frequencies for ~thich the imped~nce of the capacitor 39 is small colnpared ~/ith that of the resistor 3~t the gain t~.l be dete~.ined b~f the ratio of the resistors 37 and 38~ For the values ,0 ~i~en by ~lay Or e~3mple here.inbefore it will be seen that the galn of the arrangement will be 100 for requenc~es dotrn to a sma].1 fraction o~ 1 HæO
Certain difficulties arise if the arrangement of ~igurf~ 2 i.s employed in the amplifie~- unit 28~ If an atmospheric sample contca~nir.g 5 chemical species of the type specified i5 dra-~ through the apparatus, the resulti.ng neg~ti-~e ions reaching the electrod~ 2a till cau~e a nega~i~e going current pulse to be applied to the input terminal 27.
At the leadir.~ edge of t~ pulse, the output termi.nal 40 ttill go positi~e9 charging the capacitor 3~ At the erld of the pulse9 20 the output terminal 40 tlill not immediate'y return to its ~est ~al~e (bac~ground cu~rerlt) but t~ill be held positi~e by the cha39e on the capacitor 39. Consequently in thi.s embod.iment the apparatus can be unable to respotld to a second sanlp~e until such time as the charge on the capaci.tor 39 prcduced by a irst s~nple has leaked ~
U5ing the componer.t yalues cited, and neglecting any inte~nal leakaye in the capac.itor 39, i.t will be seen that the apparatus may req~lire se~eral hours to recove~
A sirnilar ef~ect may occ~r when the apparatus is first switc1le~
on~ when th~3 in.itial surye o current rnay leave the capacitor 39 with a positive charge, and it will be necessary to wait until this c.h~rye has leaked away before the apparatus becomes operati.onal~ ~e arrancJeme.nt of Figur~ , pro~ides a means of overcor~inq bo~h these defects~
~Irring to F:iqure 3, in ~;Jhich lntegers already descr.ibed a.re .3~3a4~
accor~ed the sc~me reference num2rals ar; :in E`i.guL-e ? 9 it w.ill h~
seen that the ol)erati.onal ~Inpli.fier 3~, the resls'Lors 37, 3~3 an~ 41 ~nd t~? caE~dci.t.or 3'J are co~ectAd as descri~ed with referen~e to Figure 2~ The common rail 36 is conr~ect~d to ~he negative bu~ o~ a d.c. supply via a zener rlic~e ~3~ e positi.Ye s~pply tel-minal of the ~mpl.ifier 34 is connected to the posi.ti~e bus 44 of the supply via a s~;itch 45~ The co~non rail. 369 and with i.t the input 33 oF the amp3.ifi~r 34 is therefore he].d positive 1:o the negatit~e bus ~2 by the~ ~ener Y~ltage Vl of the diocle 43 (tvpically 5V)~ The bus 42 is comlected to ground and to the tube 3 (E`i~lr~ l). Thus the electrode 24 is helcl at a positive potential Vl with respect to the tube 1 and the zener diode 43 comprises the voltaye source 23 o~ Figuxe 14 A fl~ther amplifier ~6 has its inve~ti.n~J input 47 connected to the output terminal 40 of the c-~pli.~iex 34~ A non~in~ertiny input 48 of the arnplifi.er 46 i5 connect.ed via a. r~sistor 49 to the j~Cti.011 50 of the resistor 38 and the c.:dp~citor 39.
A diode 51 is connected het~een the outp~t. terr~linal 52 of the amplifier 46 and .its non~invertiny input 4~, the arran~ement beiny such that ~.~hen the input 47 is posit:ive rel~tive to the illpUt 48 the diode 51 i~s non~conducting. When the input 4~ is positive relative to the input 47, i.~e~ the capacitor 39 is charyed positi~e rclatiYe to the output 40 o~ the c~nplifier 34 9 the cliode 51 is conductiYe and the capaci.tor ~9 iS ~lscharged via the resistor 49, which typi.cally has a value of: 33K~ oh~ Although the capac.itor 39 b~comes charged when a n~ative~go.ing current pulse is applied to the input terminal 27~ it now rapldly becomes dischc~ged aft~r the end of the pulse~ so that the apparatus can respond to a ~urther s~nple~
~o prevent the capacito.r 3~ h~comi.ng cha~ged when the switch 45 is first cl.osed, a p n~p transistcr: 53 is p~ovided, wlth its collector connected t.o the junction SO. T}le base of the transi.stor 53 is connectc-d via a resistor 5~ t:o the junction 55 of a resistor ~6 and a capaci-tor 57 whicl-~ are conrleted in series between the po~;:itlve bus 44 and the negci~:i.ve. bu.~ ~ 2 0 ~ res.is~.or ~3~
58 is connect~l bet~een the posi.tive bus ~4 and the emitter of the transistor 53. A diode 5~ S its an~YIe conne~cte~d to che emitter of the transistor 53 and it:s cathode to t}~e common rall 36.
~.~en the switch ~5 is closed, thee junction 55 and with i.t the base of the transistor 53 will init.i.ally be at the potential o~ the negative bus 42~ e ~ransistor 53 ~lill there~ore co~ciuct and will clamp the ~nct.ion 50 at the potential o the con~!non xail 36.
me potenti.al of the junction 55 will ri.se ~s the capacitor 57 charges v.la the resi.s-tor 56, eventu~ cut-ting ofr the transistor - 1,0 53 and .so xem~i.n~j the clarnp from the junction 5C.'2 Th~ Yalues of the resistor 56 and the CapacitQr 57 are chosen so tllat the trans~stsr 53 remà:~ns conduc~tive at least until the ampll*ier 34 has reache.d its steady~state conditi.on follo~ing the c].osure Oc the sw.itch 45, thus preventirlg the accumulation of any cha~ge on the~
c~apacitor 39 due 'o switching~on urges~
To facilitate se~ting up o* the ste~d~~st~te condi.tio~l of the~
amp~.ifier unit (hacl~ground currer~t) a po~enti.ornete~ 6S~ may be provided ~ One end of the~ potentiometer m~y be connected to the pos.itive bus ~ia the resistor 5~ and the otller end to the common 20 rail 36 via a resls~or 61, theS res.isto~s 58 ancl 61 serving to restri.ct the ~o1tac3.r-.~ e~Y.cut~slon t~Jhich rn-~f he oht.a.ired by adjust!n~nt.
of the position of the slider 62. 'l~e sli.der is connected via a resistor 63, typically of 33 M.o}~n to the junc-ti.on o~ the feeclback resistor 41 ~nd the rt-~sistors 37 and 33. By adjustmen~ of t~.e potentiometex 60 Ole backc~round current level may be set to a de~i~ed valueO
~ ne neqatlve ~2) and ~e positi~!e (fi4) buses ma}~ be connected to coxLespondil)g tenninals o~ a battery 70 o fOL eY~mple 12V
output~ m e battery 70 ~ay also be employed to enercJ.i.s2, by the on-off switch ~5, the motor 14 ar.d ~he voltage sources 23 and 32 tF.igure 1). The voltatJe source 23~ which provides the hic~
voltage neces.sary to produce the corGna discharge i.n the region 15, may conveniently comprise an ~,HI' generator circuit such as is hell kno.~l to those skilled in the art, ant~ need not be described 35 furtheL hex^e1xls Siml:Lcirly t:he~ volt2~cJe source 32 may- comprise an ~3L3~
~.~
E!ll` generator or d~c. to ~Cr eonverter of known type.
InvesticJations haYe sl^lown that ~h~ .influence o~ e~ternal air curr~nts i5 cJt.-eatly retiuce(i if the sample is eYLhau.stecl to atmosphere i.n a re~on adjclcent t-o, md subject to substanti.ally the same exter~l condltions as, the lnlet nozzle and to this encl the body 1 is surroundecl by an outer casiny 64~ ~le casing 64 has an end wall 65 spacecl fro;n the open outlet end o~ the tube 3 so as to provide a passac;eway ~or the flo~J of air e~austed by the -fan 13 into an amlular space 66 between the body 1 and the outer casi.ng 64 and thence ~o atmosphere via an annular aperture 67 surro1md:i.ncj the inlet ~nd o the body 19 ThC~ 10w path of the exhaust a.ir i.s i~icated by the arrows 63, ~9.
Convenient:Ly9 the battery 70, the voltac~e sources 23 e~nd 32, the arnplifier unit 28 and the alarm un:i.t 31 may be located wlthin the outer casincJ 6~ to produce a po.rtable~ ha~ld~held apparatu.s.
In an alternative arrangeln~nt, pri.nary ionisation may be caused by a sou~ce of ionising radiation (22' Yic~ure 4~ e~g~ a ~--parl:icle emltt.er such as ~mericium 241, or B~-particle emitters s~lch as Nlckel G3 or Tritium, located i~ a cont~ er 20' (Figure 4) with:in 2U the re~ion 15. Sine the prl~ary ion population so produced would be bi-polar, it is also necessary to protide in the reglon 15 an elcct~ic fleld effecti~e to select ions of one polarity for onward travel with the ai.r 10w and to prevent .ions o the opposit.e polarity rom approach.ing the septu~ 9. One method o~ achievi.ng this is to i.nsulate J~he sept~ 3 ~ uxe ~) by a ring 41 of insul.ating materi.al from the tube 2 and to apply a suitable potential tn rebetween~
The ~eptum 9 (Figure 1) is also insulated f.rom tube 2 when the pri.mary ionisation is caused by a so~ce o ioni.si.ng radi.ation located i.~ a container (nGt shown) in the region 15 The electrocle 20p lead 21, insulator 22 and supply unit 23 ~ay then be omitted~
The contai.ner 20' containing a sou~ce of ionisi.ng rad.iation m~y be in~erted into the re~ion 15 through the ape~ture in the wal.l 2 ~Fiyure 1.) indicated for t:he .insulator 22~ A s~,~ ~ arrange~c-:nt can be used in the app~ratu~.represented by Ficlure 4 and in this case the aperture (not sh~l:rl) wi.~l pass thro~lg7l ~oth w~11 2 a~d wall 6~o ..... .. .
~3~
'rlliS ar~ally~em~llt miti~ates problems of p.re~enting le~ka~e of atomic radiati.on f rom the app,~ratusO
~ pp.li.cations o~ the present lnventlon include~ antJ.~terroList measures such ~AS the screening for explo.siv~ substances of travellers and luggage at airway terminals and similar locations, ancl searching for explosive substances gener-ally. ~pparatus e~mbodying the invention m.cly also be empl.oyed as a detecto.r in yas chromatography.
A furthe~ application is ill the tracin~ ~f lealcs in equ-~pr;,ent such as pip~wor~ pressure vessels1 chemical plant and the like, wherein a tr~cex: sl~bstanc~ such AS a heavy halogen is intr~luced .~.nto the equipr,knt under test and the appara'cus ls rno~ed over the e.~terior of the ec~ Tnent to de-tect any escape of the trAcer substanceO
In a t.~pic.al e~:p2riment with an en~odim2nt o-f the appaLatUS
as described with re~rence to F~gure 1, the cliameter of the jet or.ifice 10 was approximately 6 mm and the rate of ~as 10w through the apparatus was approxima~ely 10 litres~nlin~te. The potential V3 o the corona di.schar~e electrode 20 was 3~Y, th~ p~tenti~-nl Vl of the collector electrode 2~ was SV and the strength o the repulsive electrlc field in the rec;i.on ].6 was 300VJcm. 'rhe resistor 71 was shorted out~ so that ID was in the range 2~5 micro-c~mps o ~ ' , Whel~ nor~al atmospheric air was clra~n through the apparatus the currer~t collected by the electrode ~ SiOe~ the back~round current) was 1 picoamp.
25. A sample o a nlixture o nitroglycerine vapour in air, havincJ a known concentration of 1 part by welght of ni~roglycerinevapour in 1~ parts by weight of the mixture was then drat~n through the apparatus9 and t.he current of the colJ.ect:or electrode 2~
then became S pico ~ps, the rise of ~ picoarrlps bei.ng caused by t}~e presence of the nitrocJlycerine ~apGllr i.n the sample~
me experiment was then rep~ated Wit}l the resisto~ 71 having a ~alue of Gi.go~ms in circuit so th~ the discharc3e current did no~ exceed 3C~ nanoanlps.
~ le back.cJround cur~erlt was then 0~3 picoamp, and the current when the nltrog~ ri.ne sam?le was drawn thrGugh the apparatus was loO picc)amp~
The reduc-t,iorl in background current i5 due to the lesser noise produced by -the limited current corona dis-charge.
In a further series of experiments, sampl~s of various obtruding substances and also samples of various exp],osives were presented, firs-tly to an apparatus in which the resis-tor 71 was shorted out and secondly to an apparatus in which the resistor was in circuit and had a value of 50 Gigohms. All other parameters of the apparatus were main-tained constant throughout the experiments. The resultsobtained are shown in the following table, in which read-ings of the meter 30 are listed against the substances.
The meter had an arbitrary linear scale from 0 to 100 (= full scale deflection).
Substance R = 0 R = 50 Gigohms Solvents PC~ O O
Methanol 0 0 Ethanol 25 8 Xylene 90 Nitrobenzene 25 0 White Spirit 75 0 _olishes Mansion* 35 0 Cherry Blossom Shoe* 75 0 Kiwi Shoe* 75 0 Brasso* 90 Turtle Wax Car Polish* 90 0 Treble Wax Car Polish* 50 0 Duraglit* 99 Toiletries Avon Blue Blazer After-Shave* 15 2 Nail Varnish 75 0 Tabac Eau de Cologne* 10 0 Chanel No. 5 Eau de Cologne* 35 0 Just Musk Perfume* 0 * trade mark products Substance R=0 R=50 Gigohms . .
Toiletries Indian Summer Perfume* 0 4 Azuree Perfume* 35 0 5Miss Balmain Perfume* 25 0 Scandale Spray* 5 0 Old Spice After-Shave* 5 2 Brylcreem* 50 3 Nivea Cream* 10 0 10Old Spice Talc for Men* 25 0 Brut After-Shave* 50 0 Sunsilk Conditioner* 35 2 Rears Hand Cream* 25 0 Ultra-Brite Toothpaste* 90 3 15Disinfectants Savlon* 70 0 Pine*
Miscellaneous ._ Petrol 15 0 20Tin of paint (Polyurethane~ 90 16 Horseradish Sauce 15 0 Ground Almonds 5 0 Curry Powder 0 0 Chanel No. 5 on Skin Wet* 25 0 " " " " " " after 5 mins 50 2 " " " " " " " 15 mins 35 5 ll ll ll ll ll ll " 25 mins 50 4 Explosives Needle from Geligni-te flask 75 99 Double-Based Powder 90 80 Plastic Igniter Cord (Slow) 25 0 Siesmex H.E.* 15 0 Polar Ajax* 75 85 Minespex* 50 36 *trade mark products ~31~V~
Substal~ce R = OR = 50 Gigohms Wincoal (A(* 50 90 Water Gel O O
. _ . .
Temperature: 25C
All samples offered as opened con-tainers (except where stated).
Applications of the present invention include, the screening for explosive substances of travellers and lug-gage at airway terminals and similar locations, and search-ing for explosive substances generally. Apparatus embody-ing the invention may also be employed as a detector in gas chromatography. A further application is in the tracing of leaks in equipment such as pipework, pressure vessels, chemical plant and the like, wherein a tracer substance such as a heavy halogen is introduced into the equipment under test and the apparatus is moved over the extexior of the equipment to detect any escape of the substance.
* trade mark products
Claims (14)
THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. An apparatus for detecting trace quantities of che-mical species in the form of a vapour contained in an oxy-gen-containing gaseous mixture by subjecting ions produced in a sample of the mixed gases under flow to an ion repul-sive electric field, said ion repulsive field being direc-ted in the opposite direction to the flow of the gases characterised in that there is provided. means for drawing the sample through. a hollow body, said hollow body having serially arranged an inlet, a first, a second and a third internal region and an outlet, the first region containing means for ionising at least a proportion of the molecules of the sample inlcuding molecules of the chemical. species and means for selecting ions of one polarity for travel with the sample into the second region, means for promoting the flow of the sample through the second region as a jet of substantially uniform velocity, means for producing in the second region the oppositely directed ion repulsive electric field and means in the third region for collecting ions of selected polarity, the arrangement being such that, in operation, ions of the one selected polarity whose ionic mobilities exceed a value dependent on the strength of the ion repulsive electric field and the velo-city of the gas flow can be prevented from entering the third region.
2. An apparatus according to Claim 1 in which the hollow body comprises first and second tubes of an electrically conducting material joined in end to end relationship by a ring of insulating material, said first region being located within the first tube and the second and third regions being located within the second tube.
3. An apparatus according to Claim 2 in which the first tube is provided at its end remote from the second tube with an end wall having an inlet aperture for admit-ting the sample to the first region and at the end adjacent to the second tube with a disc of an electrically conducting material, said disc being positioned transverse to the gas flow and having at least one aperture for admitting the sample to the second region.
4. An apparatus according to Claim 3 in which the or each aperture of the disc at the end adjacent the second tube is shaped so that the sample passes through the second region in a stream-lined manner and with a sub-stantially uniform velocity.
5. An apparatus according to Claim 3 in which the second and third regions are partially separated by a fur-ther disc of an electrically conducting material located at a point remote from the ends of the second tube and having at least one aperture leading from the second region to the third region for admitting the sample of the third region, said further disc being in electrical contact with the wall of the second tube.
6. An apparatus according to Claim 2 in which the means for ionising molecules of the sample including molecules of the chemical species is an electrode located in the first region and electrically connected, in use, to a high direct voltage electrical supply whereby to produce a corona dis-charge in the first region and also to produce an electric field for selecting ions of one polarity for travel with the sample into the second region.
7. An apparatus as claimed in Claim 6 comprising means for limiting the corona discharge current to not more than 100 nanoamps.
8. An apparatus according to Claim 2 in which the means PP. 1216C
for ionising molecules of the sample including molecules of the chemical species is a radioactive source located in the first region and in which the disc between the first and second regions is electrically insulated from the wall of the first tube and is maintained at a direct electric poten-tial with respect to the said wall so that ions of one polarity are attracted towards the disc and ions of the opposite polarity are attracted towards the wall.
for ionising molecules of the sample including molecules of the chemical species is a radioactive source located in the first region and in which the disc between the first and second regions is electrically insulated from the wall of the first tube and is maintained at a direct electric poten-tial with respect to the said wall so that ions of one polarity are attracted towards the disc and ions of the opposite polarity are attracted towards the wall.
9. An apparatus according to Claim 5 in which the fur-ther disc is maintained at an electrical potential relative to the first disc so as to produce an electric field in the second region which produces a force on the ions of the one selected polarity in a direction opposite to the flow of the sample.
10. An apparatus according to Claim 2 in which the means for collecting ions of the one selected polarity includes an electrode provided in the third regions of the hollow body, said electrode being electrically connected to a source of direct potential to attract the ions of selected polarity and to the input of a current amplifier of which an output is connected.
11. Apparatus as claimed in Claim 10 wherein the ampli-fier includes feedback means connected between an inverting input of the amplifier and a tapping point on a potentio-meter chain, which potentiometer chain is connected between an output of the amplifier and a common rail and includes a capacitor connected between the tapping point and the common rail whereby the gain of the amplifier has a low value for a substantially constant input current and a higher value for a changing input current.
12. An apparatus according to Claim 11 including means for comparing the potential on the capacitor with the potential at the output of the amplifier and means for dis-charging the capacitor when its potential exceeds that at the output of the amplifier.
13. An apparatus according to Claim 11 including switch means effective when the apparatus is switched on to con-nect a low impedance path across the capacitor and to dis-connect the said path when the power supply to the apparatus PP. 1216C
has reached a steady state.
has reached a steady state.
14. A method for detecting trace quantities of chemical species in the form of a vapour in an oxygen-containing gaseous mixture by subjecting ions produced in a sample of the mixed gases under flow to an ion repulsive electric field, said repulsive field being directed in the opposite direction to the flow of gases characterized by the steps of drawing the gaseous mixture through the hollow body as defined by Claim 1 wherein a proportion of the molecules of the chemical species are ionised in the first region and ions of one polarity are selected for onward travel with the sample of the gaseous mixture, to pass with the gas flow at a substantially uniform velocity through the second region wherein is established an electric field effective to urge ions of the selected polarity in a direc-tion opposite to that of the gas flow and through to a third region to which the electric field does not extend, and detecting ions whose ionic mobilities are sufficiently low so that they are carried by the gas flow through the electric field in the second region.
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB2304078A GB1573678A (en) | 1978-05-26 | 1978-05-26 | Trace vapour detection apparatus |
| GB23040-78 | 1978-05-26 | ||
| GB2476478A GB1596916A (en) | 1978-05-31 | 1978-05-31 | Trace vapour detection |
| GB24764-78 | 1978-05-31 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1138044A true CA1138044A (en) | 1982-12-21 |
Family
ID=26256273
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000323634A Expired CA1138044A (en) | 1978-05-26 | 1979-03-15 | Trace vapour detection |
Country Status (5)
| Country | Link |
|---|---|
| US (1) | US4271357A (en) |
| EP (1) | EP0004124B1 (en) |
| JP (1) | JPS54157688A (en) |
| CA (1) | CA1138044A (en) |
| DE (1) | DE2963359D1 (en) |
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| US4828800A (en) * | 1983-08-15 | 1989-05-09 | Brunswick Corporation | System for trace gas detection |
| FI75055C (en) * | 1986-06-03 | 1988-04-11 | Puumalaisen Tutkimuslaitos Oy | Method for observing gas content of ingredients. |
| US4686918A (en) * | 1986-06-10 | 1987-08-18 | Schlegel Corporation | Method and apparatus for making tufted buffing pads of varied density |
| FR2604257B1 (en) * | 1986-09-19 | 1993-02-05 | Masuda Senichi | INSTRUMENT FOR MEASURING ULTRA-FINE PARTICLES |
| GB8625593D0 (en) * | 1986-10-25 | 1986-11-26 | Analytical Instr Ltd | Ion drift detector |
| US4973909A (en) * | 1988-09-16 | 1990-11-27 | University Of Western Ontario | System for the measurement of the concentration particulates in a flowing gas |
| US5109691A (en) * | 1989-12-08 | 1992-05-05 | Research Corporation Technologies, Inc. | Explosive detection screening system |
| GB9127557D0 (en) * | 1991-12-31 | 1992-02-19 | Secr Defence | Explosives detector |
| GB9510405D0 (en) * | 1995-05-23 | 1995-07-19 | Graseby Dynamics Ltd | Ion mobility spectrometers |
| GB9602158D0 (en) * | 1996-02-02 | 1996-04-03 | Graseby Dynamics Ltd | Corona discharge ion sources for analytical instruments |
| JP2000028579A (en) * | 1998-07-08 | 2000-01-28 | Hitachi Ltd | Sample gas collecting device and hazardous substance detecting device |
| US6630663B2 (en) * | 1998-10-21 | 2003-10-07 | Raytheon Company | Miniature ion mobility spectrometer |
| IT1319850B1 (en) * | 2000-02-18 | 2003-11-03 | Viro Tronic S P A | ELECTRONIC DEVICE FOR THE DETECTION OF NARCOTIZING GASES. |
| US6412447B1 (en) | 2001-04-16 | 2002-07-02 | The Water Heater Industry Joint Research And Development Consortium | Fuel-fired water heater with flammable vapor sensor and associated induced flow tube |
| US6630662B1 (en) * | 2002-04-24 | 2003-10-07 | Mds Inc. | Setup for mobility separation of ions implementing an ion guide with an axial field and counterflow of gas |
| US7095019B1 (en) | 2003-05-30 | 2006-08-22 | Chem-Space Associates, Inc. | Remote reagent chemical ionization source |
| WO2004092705A2 (en) * | 2003-04-09 | 2004-10-28 | Brigham Young University | Cross-flow ion mobility analyzer |
| US7138626B1 (en) | 2005-05-05 | 2006-11-21 | Eai Corporation | Method and device for non-contact sampling and detection |
| US7568401B1 (en) | 2005-06-20 | 2009-08-04 | Science Applications International Corporation | Sample tube holder |
| US7576322B2 (en) * | 2005-11-08 | 2009-08-18 | Science Applications International Corporation | Non-contact detector system with plasma ion source |
| FI119660B (en) * | 2005-11-30 | 2009-01-30 | Environics Oy | Method and apparatus for measuring ion mobility in a gas |
| US8123396B1 (en) | 2007-05-16 | 2012-02-28 | Science Applications International Corporation | Method and means for precision mixing |
| US7709787B2 (en) * | 2007-08-24 | 2010-05-04 | The United States Of America As Represented By The Secretary Of The Department Of Commerce | Stepped electric field detector |
| JP4445996B2 (en) * | 2007-12-14 | 2010-04-07 | 株式会社日立製作所 | Ion mobility spectrometer |
| US8008617B1 (en) | 2007-12-28 | 2011-08-30 | Science Applications International Corporation | Ion transfer device |
| US8502507B1 (en) | 2012-03-29 | 2013-08-06 | Accio Energy, Inc. | Electro-hydrodynamic system |
| EP2238678B1 (en) | 2008-01-22 | 2015-12-16 | Accio Energy, Inc. | Electro-hydrodynamic wind energy system |
| US8878150B2 (en) | 2008-01-22 | 2014-11-04 | Accio Energy, Inc. | Electro-hydrodynamic wind energy system |
| US8071957B1 (en) | 2009-03-10 | 2011-12-06 | Science Applications International Corporation | Soft chemical ionization source |
| CN103081338B (en) * | 2010-10-18 | 2016-06-29 | 阿齐欧能源公司 | For controlling the system and method for electric field at electricity-flow power in applying |
| US20140370613A1 (en) * | 2011-12-14 | 2014-12-18 | Waters Technologies Corporation | Atmospheric Pressure Chemical Ionization Detection |
| JP6138652B2 (en) * | 2013-10-01 | 2017-05-31 | 日本特殊陶業株式会社 | Fine particle measurement system |
| EP3225982A1 (en) * | 2016-03-28 | 2017-10-04 | Naturion Pte. Ltd. | Device for measuring ion concentration |
| JP7251307B2 (en) * | 2019-05-16 | 2023-04-04 | 株式会社リコー | Analysis system, fecal odor gas analysis system and breath gas analysis system |
| US11353381B1 (en) * | 2020-06-09 | 2022-06-07 | Applied Materials, Inc. | Portable disc to measure chemical gas contaminants within semiconductor equipment and clean room |
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| NL224264A (en) * | 1957-10-24 | |||
| GB1105604A (en) * | 1964-04-25 | 1968-03-06 | Arthur Edward Caswell | Determining the concentration of particles suspended in air |
| DE1679532B1 (en) * | 1967-10-09 | 1970-12-10 | Berckheim Graf Von | Arrangement for generating unipolar air ions |
| US3522425A (en) * | 1967-11-28 | 1970-08-04 | Gen Electric | Apparatus for selective separation of ions of different mobilities in a gas stream |
| US3668385A (en) * | 1969-10-13 | 1972-06-06 | Franklin Gno Corp | Apparatus and methods for improving measurements performed upon gaseous samples by reducing sample contamination |
| DE2028805C3 (en) * | 1970-06-11 | 1974-05-22 | Franklin Gno Corp., West Palm Beach, Fla. (V.St.A.) | Method and device for determining a gas component |
| DE2346422A1 (en) * | 1972-09-14 | 1974-04-04 | Stanford Research Inst | METHOD AND SYSTEM FOR DETECTION OF EXPLOSIVE SUBSTANCES |
-
1979
- 1979-03-08 US US06/018,801 patent/US4271357A/en not_active Expired - Lifetime
- 1979-03-14 EP EP79200128A patent/EP0004124B1/en not_active Expired
- 1979-03-14 DE DE7979200128T patent/DE2963359D1/en not_active Expired
- 1979-03-15 JP JP2944679A patent/JPS54157688A/en active Granted
- 1979-03-15 CA CA000323634A patent/CA1138044A/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| JPS6125095B2 (en) | 1986-06-13 |
| US4271357A (en) | 1981-06-02 |
| JPS54157688A (en) | 1979-12-12 |
| EP0004124A1 (en) | 1979-09-19 |
| DE2963359D1 (en) | 1982-09-09 |
| EP0004124B1 (en) | 1982-07-21 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |